For as long as humans have been on this planet, the microscopic life forms that we call germs, or pathogens, have been here too. For all this time they have been multiplying in the soil, in the water, and in our bodies. They are in the mouth, on the skin, and even in important organs. But that is not always a bad thing, because humans could not live without germs. Some germs, or “microbes” help digest food, while others produce vitamins. Most of the time, people and germs coexist peacefully; in fact there are trillions of germs on you right now.
Even though some germs are good for us and help us, others can give us diseases. Some diseases only make us sick for a period of time, while others cause incurable and deadly illnesses. Such diseases as cancer and diabetes result mostly from environmental conditions or inherited weaknesses. They are considered noninfectious diseases because one person or animal cannot make another one sick.
For a long time, people thought germ-born illnesses could be conquered by science. In the late 1900s, new vaccinations and drugs seemed to have them on the run, but germs have bounced back. The drugs that once worked lost their punch, while new diseases keep cropping up. Medicine has made huge strides in fighting germs, but Earth’s tiniest creatures remain our biggest foes.
Germs are often called pathogens, which means “the causes of disease,” in Greek. The four main types of pathogens are bacteria, viruses, parasites, and fungi. A bacterium is a simple one-celled organism. They are like us because their goal is to survive and prosper. Bacteria are three and a half billion years old. They are the most numerous living thing, and among the smallest. Bacteria can reproduce very fast. One bacterium can multiply into thousands in just a few hours. They attack by destroying healthy cells or releasing poisons. Bacteria can cause a wide range of illness, such as strep throat, cholera, leprosy, pneumonia, tuberculosis, typhoid fever, whooping cough, diphtheria, scarlet fever, tetanus, botulism, and anthrax. Most bacteria can be stopped by antibiotics, but a large number of them can be resistant to drugs.
A virus is a lifeless particle that is shaped like rods and spheres. The biggest virus is the size of the smallest bacterium. These tiny organisms can not move on their own. They are carried in water, food, wind, blood, and other body secretions. It springs to life after latching on to a living cell, then it invades the cell and creates more viruses. Like bacteria, viruses multiply quickly. Viruses can cause influenza, common cold, measles, mumps, rubella, polio, herpes, smallpox, AIDS, chicken pox, rabies, and yellow fever. When available, vaccines are the best defense.
Parasites are one-celled organisms that look like worms. They can also be called protozoa. Like viruses, protozoa must invade suitable hosts in order to complete their life cycle. They are more complex than bacteria and viruses. When the worms start off, they are microscopic, but some can get up to twenty feet long. Parasites can cause malaria, dysentery, tapeworm, hookworm, elephantiasis, guinea worm, and pinworm. Parasites can be very deadly. Though most are treatable, parasites are usually painful and cause other health problems.
A fungus is an organism that tends to like warm, damp places. About 182,000 out of 250,000 known species of fungi are able to cause disease in humans. Fungi have ridged cell walls, which makes them immobile. Some fungi are microscopic, while others can be seen by the naked eye. Fungi can cause athletes foot, ringworm, thrush, and other various skin disorders. Fungi can also produce serious diseases in plants and animals. It is seldom fatal to humans unless the immune system is weakened. Together, bacteria, viruses, parasites, and fungi have killed and injured more people than all the worlds’ wars combined.
Infectious diseases are the third leading cause of death in North America, but they are the number one killers in the rest of the world.
The top ten deadly infectious diseases in 2001 were in the following order:
1. Respiratory Infection (including flu) 3.8 million died
2. HIV/Aids, 2.9 million died
3. Diarrheal diseases (including cholera), 2 million died
4. Tuberculosis, 1.6 million died
5. Malaria, 1.1 million died
6. Measles, 745,000 thousand died
7. Whooping cough, 285,000 thousand died
8. Tetanus, 282,000 thousand died
9. Meningitis, 173,000 thousand died
10. Syphilis, 167,000 thousand died
These infectious diseases can be spread in a frightening variety of ways; through the air, from person to person, by touching infected material, through a healthy carrier of the disease-causing organism, by animals including household pets, by insects such as mosquitoes, fleas and ticks, and infected water or food. The manner in which a disease is spread depends on the particular organism causing the disease.
Germs have shaped human life in many dramatic ways. Around 9,000 B.C., humans became farmers. Because of close contact with farm animals, the number of human diseases exploded. Measles come from dogs, flu comes from pigs, anthrax from sheep, and tuberculosis from cows. People in tight knit farm communities spread these illnesses easily. Around 1347, the Black Death spread from Asia to Africa and Europe. Within five years it killed at least one third of all Europeans. In 1492, Christopher Columbus landed in the new world. His crew and later arrivals brought diseases such as Small pox with them. Native Americans had no immunity, and between 50 and 90 percent of them died, making way for Europeans to conquer North and South America.
In 1796, an English doctor discovered the first vaccine. In the 1860’s, French chemist Louis Pasteur discovered that bacteria cause illnesses. He calls bacteria, germs. His germ theory of disease becomes the basis of all disease fighting. In 1900, Army Major Walter Reed, proved that mosquitoes transmit yellow fever. The mosquito-control effort allows the Panama Canal to be built through dense jungles. In 1918 a strong strain of influenza, or flu, kills more than 20 million people worldwide. That is about five million more deaths than occurred in World War 1.
During 1928, Scottish scientist Alexander Fleming noticed that a mold called penicillium kills bacteria. This leads to the discovery of the first antibiotic, which makes bacterial diseases such as tuberculosis treatable for the first time. In 1946, the United States forms the Communicable Disease Center. It becomes the nations disease watchdog and helps lead worldwide battles against smallpox, polio, and other illnesses.
In 1980, a global vaccination program leads to the eradication of the virus that causes smallpox. It is the only time that a germ has successfully been made extinct. But some countries may have kept a sample to use in biological warfare. In 1981, a mysterious new disease begins to kill people in the United States, Europe, and Africa. It becomes known as acquired immune deficiency syndrome, AIDS. In April 2003, a businessman in Foshan, China, falls ill with a mystifying type of pneumonia. Within three months, severe acute respiratory syndrome or, SARS has spread around the world. Efforts to contain, the contagious virus proves difficult. A new killer is born.
Major epidemics of our past have killed millions of people, destroyed empires and won wars. Smallpox is one of the greatest scourges in human history. This disease, which starts with a distinctive rash that progresses to pus-filled blisters and can result in disfiguration, blindness, and death, first appeared in agricultural settlements in northeastern Africa around 10,000 B.C.E. Egyptian merchants spread it from there to India. Evidence of smallpox skin lesions has also
been found on the faces of mummies from the eighteenth and twentieth Egyptian dynasties.
The first recorded smallpox epidemic occurred in 1350 B.C.E. during the Egyptian-Hittite War. In 430 B.C.E., the second year of the Pelonponnesian War, smallpox hit Athens and killed more than 30,000 people, reducing the population by 20 percent. Athens was the only Greek city hit by the epidemic, but Roe and several Egyptian cities were affected. Smallpox then traveled along the trade routes from Carthage. In 910, Rhazes, (a Persian doctor) provided the first medical description of smallpox, documenting that the illness was transmitted from person to person.
The patterns of disease transmission often paralleled peoples’ travel and migration routes. Disease in Asia and Africa spread to Europe during the Middle Ages. Smallpox was brought to the Americans with the arrival of Spanish colonists in the fifteenth and sixteenth centuries, and it is widely acknowledged that smallpox infection killed more Aztec and Inca people than the Spanish Conquistadors, helping destroy those empires.
Smallpox continued to ravage Europe, Asia, and Africa for centuries. In Europe, near the end of the eighteenth century, the disease accounted for nearly 400,000 deaths each year, including five kings. Of the surviving, one-third were blinded. The worldwide death toll was staggering and continued well into the twentieth century, where mortality has been estimated at 300 to 500 million. This number vastly exceeds the combined total of deaths in all world wars.
In the United States, more than 100,000 cases of smallpox were recorded in 1921. Strong declines occurred after that because of the widespread use of preventive vaccines. By 1939, fewer than 50 Americans per year died of smallpox.
In 1959, the World Health Assembly decided to organize mass immunization campaigns against smallpox. The (WHO) announced the global smallpox eradication program in 1967. At
that time there were still an estimated 10 to 15 cases of smallpox a year resulting in two million deaths, millions disfigured, and another 100,000 blinded. Ten years later, after dispersal of 465 million doses of vaccine in 27 countries, the last reported naturally occurring case appeared in Somalia. On October 22, 1977, a 23 year old male, Ali Maow Maalin, developed smallpox and survived. The global campaign against smallpox ended in 1979 just two years after Maalin’s case. Two additional cases of smallpox occurred in Birmingham, England, in 1978, after the virus escaped from a laboratory. There has not been a case reported in more than 25 years.
HIV/AIDS, is the birth of a modern raging epidemic. On June 5, 1981, Centers for Disease Control reported five young men, active homosexuals who were treated for biopsy-confirmed Pneumocystis carinii pneumonia. All five patients had laboratory-confirmed cytomegalovirus (CMV) infection and yeast infections. Pneumocystis pneumonia in the United States is almost exclusively limited to severely immunosuppressed patients. The age of AIDS had begun.
It soon became clear that an infectious agent, probably a virus, was spreading through blood. By the end of 1981, 422 cases were diagnosed in the United States and 159 people were dead. In 1982, the number rose to 1,614 cases diagnosed and 619 dead. The same year, the acronym GRID was replaced with AIDS for “acquired immune deficiency syndrome” because the disease, which is acquired from someone else, results from an inability or deficiency of the immune system to work properly. By early 1983, evidence was building that the agent causing AIDS could be transmitted sexually, as well as through blood and blood producers. At the end of 1983, 4,749 cases were reported and 2,122 were dead.
As the epidemic grew, there was serious concern about the safety of the blood supply. People who received transfusions and hemophiliacs receiving a specific blood-clotting factor were getting AIDS. Unfortunately, health officials miscalculated the magnitude of the problem
and during the early 1980’s almost half the 16,000 hemophiliacs in the United States contracted AIDS. It was not until 1985, after HIV, the cause of AIDS, was isolated and characterized that an antibody-based test to determine whether blood was safe was developed and approved. Much has been written about this sad early chapter of the AIDS epidemic.
In the early and mid 1990’s, an education program in North America and Europe, stressing the need for protection from bodily fluid transfer, helped to slow the spread of AIDS. Yet around the world, the number of AIDS cases continued to rise. In 2001, an estimated 40 million people, 37.2 million adults and 2.7 million children, were living with HIV. Twenty years after the first cases were reported, a modern epidemic was ravaging the world’s poor.
Most striking are the 28.1 million AIDS cases in Africa, with most in the sub-Saharan Africa. Nearly 2.3 million Africans died from AIDS in 2001. Other countries, like India, are experiencing meteoric rises in AIDS cases. Some countries with little history of AIDS, like China and Russia, are poised for a major epidemic as well.
The human body is built to fight off germs. We are exposed to thousands and thousands of infectious agents every single day. Most of us only get sick from them on rare occasions. Surface barriers, like our skin and mucous membranes, are the body’s first line of defense. If the foreign invader gets past the first lines of defense, it might take a while for the body to figure out it has been taken hostage. Once the body gets wind of the invasion, it launches what is called an immune response, which helps to repel the invader.
The skin and linings of our digestive, respiratory, and urinary tracts are the body’s first line of defense against infection. Few germs can penetrate unbroken skin. However, they do work their way in through cuts or openings like the nose and mouth. Tears constantly wash away foreign objects. They contain their own antibiotic, an enzyme called lysozyne, which kills bacteria. The acid that is produced by the lining of our stomachs can neutralize many invaders.
The mucus, lining the inside walls of our organs and respiratory tract, prevents many
harmful bacteria from entering our system. They produce sticky mucus that traps germs. All these physical barriers produce chemicals that inhibit the growth of invaders. Bacteria that live in us all the time, and do not cause disease, compete for nutrients and attachment sites on cells. The good bacteria take all seats and leave the invaders with no place to go. Infection occurs when an invader crosses these physical barriers. Then our bodies call on the highly specialized forces of the immune system to help repel the invader.
The second line of defense is the immune system. The immune system is a group of cells in the blood and lymph (a bodily fluid). The immune response is the same in all people. It is
immunity against any diseases, and it can repel an infection or hold it in check until our body mounts a more specific adaptive response.
Once an invader enters the body several types of large white blood cells engulf infectious particles and digest them. They also secrete chemicals called cytokines, which signal the body to start the adaptive response. As cytokines are secreted, they help to increase our body temperature. A high body temperature is good for fighting off infections, because most disease-causing organisms grow better at lower temperatures, and the adaptive immune response is more intense at higher temperatures.
The cell eaters also secrete a variety of proteins and other chemicals that lead to an inflammatory response. This includes pain, redness, heat, and swelling at the infection site. Although it may be uncomfortable for us, it is the body’s way of letting us know that the fight against the infectious invader has begun. During this phase, more cell eaters are recruited to the infection sit.
Adaptive immunity is triggered when an infection escapes our defenses and makes enough of a substance called an antigen for the immune system to recognize and respond to in a very specific way. Until the infectious agent creates enough antigens for our body to recognize it, we are fighting an unknown enemy. In response to the antigen, the body makes its own specific chemicals, called antibodies that attach to antigens and inactivate them.
An antigen is a chemical produced by an infectious invader that lives on the surface of the invader and prompts the immune system to produce another chemical that can attach to it and thereby destroy the invader. An antibody is a chemical produced by the body’s immune system that can destroy invading organisms.
There are two kinds of adaptive immunity, the cellular immunity and the humoral immunity. The cellular is part of the adaptive immune response that involves the body’s production of specific white blood cells. There are two different types of white blood cells that are able to recognize and destroy microorganisms. B cells and T cells. Both B and T cells originate in the bone marrow. B cells produce antibodies. T cells attack the antigens of invaders directly and control the immune response. They are continually on the lookout for signs of disease. When the body is infected, they travel to the infection site and spring into action.
While blood cells have a short life cycle, from a few days to a few weeks. A drop of blood has 7,000 to 25,000 white cells, but that number increases if the immune system is fighting infection. White blood cells fight infection in four ways:
1. Within several days, the body manufacturers T and B cells that are made specifically to respond to the antigens of the invader.
2. T and B cells multiply and migrate to patrol the tissues of the body. They circulate in blood and in a specialized system of vessels called the lymphatic system. They come in a variety of forms that have different ways of killing invaders.
3. Cytotoxic T cells come into direct contact with infected cells and kill them.
4. Helper T cells serve a regulatory function. They are needed to activate B cells and other T cells so they can kill invaders. These helpers also turn off the T and B cells when the killing job is done.
Humoral immunity involves the production of antibodies. B cells produce antibodies that circulate in the blood and lymph streams and attach to foreign antigens to mark them for destruction by other immune cells. The humoral response is the body’s recognition of those specific antigens and its manufacture of antibodies. Antibodies work against viruses, which invade and take over the body’s own cells.
The immune system has a memory. Immunological memory is the ability of the immune system to respond rapidly and effectively to invaders it has seen before. The memory is created and stored in the white blood cells and antibodies that were produced during the first infection. These memory cells live a long time and continuously re-circulate and patrol the bloodstream. If they come in contact with a previous invader, they jump into action within 24 hours. Vaccines are created using the principles of immunological memory.
One of the best ways to fight disease is to keep people from getting it in the first place. Vaccination is one method of accomplishing that. Vaccination depends on building a person’s immunity to a particular disease-causing agent by exposing them to an avirulent (nondisease-causing) form of it without making them sick.
A vaccine is made by creating a weakened version of an infectious agent, or part of one, that can not cause a full-blown disease but that does contain antigens (the bad guys) that will elicit the creation of antibodies (the good guys) by the body’s immune system. There are a number of ways to do this. With killed or inactivated vaccines the organism is killed and then injected into the body. This type will not cause disease, so it is considered safe. Since it is not the strongest type of vaccine, booster shots may be required every few years to be sure the vaccine continues to work.
The acellular vaccine uses only part of the organism that makes antigens. The flu vaccine is an example of this. Like killed vaccines, boosters may be required, and they are safe for those with compromised immune systems. The attenuated vaccines method, weakens a live organism by aging it or altering its growth conditions. Vaccines made this way are most successful, probably because they grow in the body and cause a large immune response. This also means they are the riskiest because they can sometimes cause disease. They are not recommended for patients with weakened immune systems. Examples are measles, mumps, and rubella. Immunity usually lasts a lifetime, so no booster shots are required.
Some vaccines are made from poisons that these disease-causing organisms secrete. The toxins are chemically treated to decrease their harmful effects. Diphtheria and tetanus vaccines are toxoids. Since the immune response they induce can be weak, they are often given with an adjuvant – another agent that increases immune response. When more than one vaccine is administered together, it is called conjugated vaccine. Boosters are sometimes required every ten years.
Biotechnology and genetic engineering have been used to make subunit vaccines, which use only the parts of an organism that stimulate a strong immune response. Researchers separate the disease causing genes and then isolate and purify them to be used as a vaccine. Hepatitis B vaccine is an example of this. These vaccines are safe for those with weakened immune systems because they can not cause disease.
Vaccines work well in preventing disease in communities as well as in individuals, but vaccination is not always successful and it can not protect everyone. It is a particularly weak approach when there are many different strains of an organism causing disease, like influenza, or when the organism changes rapidly, like HIV, or against new diseases caused by unknown organisms. However, they are the best way to combat viral infections, but it is not easy to make a vaccine that is both safe and effective.
Antibiotics are substances that kill or harm bacteria. When used properly, antibiotics are casually, in response to any illness, leading to resistance later. Antibiotic resistance has become a major public health problem. It is important to use antibiotics wisely so that they remain useful to us in our fight against disease. The first antibiotics were made from naturally occurring substances.
However, over the past 50 years, researchers also develop antibiotics that are completely man-made. Synthetic drugs are usually just variations of naturally occurring molecules. They can be manufactured by chemical synthesis; they tend to be easier to mass-produce in a pure form. Naturally occurring drugs must be purified from their source in an active form, which is not always easy.
There are thousands of antibiotics, and they fall into several different classes. Each class attacks a different part of the bacteria. Once a bacterium develops resistance to one antibiotic of a certain class, it is often resistant to all others that work the same way. Some antibiotics interfere with the synthesis of the bacterial cell wall, causing a defect in the wall that makes the bacteria burst when it begins to grow and divide. Others keep bacteria from making specific chemicals needed for cell survival and cell division. Still others cause misreadings of genetic material, which interfere directly with cell division.
The most effective antibiotics are those that actually kill bacterial cells. These are called bacteriocidal drugs. Other antibiotics that do not have killing properties are called bacteriostatic, these do not kill the bugs but they do prevent the bugs from continuing to grow. Sometimes doctors prescribe a combination of drugs to get maximum effectiveness. Using both kinds of drugs is also one way to combat the development of resistance.
The optimism of the early antibiotic era has diminished. New diseases emerge, and we have seen old diseases re-emerge, sometimes in deadly new drug-resistant forms. We must come up with new ways to use existing drugs, use new combinations of drugs and develop new drugs. Unfortunately, with the speed of global travel today, a dangerous infection can spread worldwide within hours.
Courtney from Study Moose
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