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MicrobesInfectious diseases are caused by microbes which are the organisms too tiny to see without a microscope. Most microbes belong to one of four major groups: bacteria, viruses, fungi, or protozoa. Bacteria
Like humans, some bacteria (aerobic bacteria) need oxygen to survive, but others (anaerobic bacteria) do not. Amazingly, some can adapt to new environments by learning to survive with or without oxygen. Like all living cells, each bacterium requires food for energy and building materials. There are countless numbers of bacteria on Earth - most are harmless and many are even beneficial to humans. In fact, less than 1 percent of them cause diseases in humans. For example, harmless anaerobic bacteria, such as Lactobacilli acidophilus, live in human intestines, where they help to digest food, destroy disease-causing microbes, fight cancer cells, and give the body needed vitamins. Healthy food products, such as yogurt, sauerkraut, and cheese, are made using bacteria. Certain bacteria give off toxins that can seriously affect our health. Botulism, a severe form of food poisoning, affects the nerves and is caused by toxins from Clostridium botulinum bacteria. Under certain circumstances, however, bacterial toxins can be helpful. Several vaccines that protect us from getting sick are made from bacterial toxins. One type of pertussis vaccine, which protects infants and children from whooping cough, contains toxins from Bordetella pertussis bacteria. This vaccine is safe and effective and causes fewer reactions than other types of pertussis vaccine. Viruses
Unlike most bacteria, most viruses do cause disease because they invade living, normal cells, such as those in the human body. They then multiply and produce other viruses like themselves. Each virus is very particular about which cell it attacks. Various human viruses specifically attack particular cells in the body's organs, systems, or tissues, such as the liver, respiratory system, or blood cells. Although types of viruses behave differently, most survive by taking over the machinery that makes a cell work. Briefly, when a single virus particle, a "virion", comes in contact with a cell it likes, it may attach to special landing sites on the surface of that cell. From there, the virus may inject molecules into the cell, or the cell may swallow up the virion. Once inside the cell, viral molecules such as DNA or RNA direct the cell to make new virus offspring. That's how a virus "infects" a cell. Viruses can even "infect" bacteria. These viruses, called bacteriophages, may help researchers develop alternatives to antibiotic medicines for wiping out bacterial infections. Many viral infections do not result in disease. For example, by the time most people in the United States become adults, they have been infected by cytomegalovirus (CMV). Most of these people, however, do not develop CMV disease symptoms. Other viral infections can result in deadly diseases, such as HIV infection, which causes acquired immunodeficiency syndrome (AIDS). Fungi
Mycoses can affect skin, nails, body hair, internal organs such as the lungs, and body systems such as the nervous system. Aspergillus fumigatus, for example, can cause aspergillosis, a fungal infection in the respiratory system. Some fungi have made our lives easier. Penicillin and other antibiotics, which kill harmful bacteria in our bodies, are made from fungi. Other fungi, like certain yeasts, also can be beneficial. For example, when a warm liquid like water and a food source are added to certain yeasts, the fungus ferments. The process of fermentation is essential for making healthy foods like some breads and cheeses. Protozoa
Malaria is caused by a protozoan parasite. Another protozoan parasite, Toxoplasma gondii, causes toxoplasmosis in humans. This is an especially troublesome infection in pregnant women because of its effects on the fetus and in people with HIV infection or other immune deficiency.
Common Infectious Diseases
TreatmentHow an infectious disease is treated depends on the microbe that caused it and sometimes on the age and medical condition of the person affected. Certain diseases are not treated at all, but are allowed to run their course, with the immune system doing its job alone. Some diseases, such as the common cold, are treated only to relieve the symptoms. Others, such as strep throat, are treated to destroy the offending microbe as well as to relieve symptoms. For Bacteria The last century saw an explosion in our knowledge about how microbes work and in our methods of treating infectious diseases. For example, the discovery of antibiotics to treat and cure many bacterial diseases was a major breakthrough in medical history. Doctors, however, sometimes prescribe antibiotics unnecessarily for a variety of reasons, including pressure from patients with viral infections. Patients may insist on being prescribed an antibiotic without knowing that it won't work on viruses. Colds and flu are two notable viral infections for which some doctors send their patients to the drugstore with a prescription for an antibiotic. Because antibiotics have been overprescribed or inappropriately prescribed over the years, bacteria have become resistant to the killing effects of these drugs. This resistance, called antimicrobial or drug resistance, has become a very serious problem, especially in hospital settings. Bacteria that are not killed by the antibiotic become strong enough to resist the same medicine the next time it's given. Because bacteria multiply so rapidly, changed or mutated bacteria that resist antibiotics will quickly outnumber those that can be destroyed by those same drugs. For Viruses Viral diseases can be very difficult to treat because viruses live inside the body's cells where they are protected from medicines in the blood stream. Researchers developed the first antiviral drug in the late 20th century. The drug, acyclovir, was first approved by the U.S. Food and Drug Administration to treat herpes simplex virus infections. Only a few other antiviral medicines are available to prevent and treat viral infections and diseases. Health care professionals treat HIV infection with a group of powerful medicines which can keep the virus in check. Known as highly active antiretroviral therapy, or HAART, the new treatment has improved the lives of many suffering from this deadly infection. Viral diseases should NEVER be treated with antibiotics. Sometimes a person with a viral disease will develop a bacterial disease as a complication of the initial viral disease. For example, children with chickenpox often scratch the skin sores caused by the viral infection. Bacteria such as staph can enter those lesions and cause a bacterial infection. The doctor may then prescribe an antibiotic to destroy the bacteria. The antibiotic, however, will not work on the chickenpox virus. It will work only against staph. Unfortunately, safe and effective treatments and cures for most viral diseases have eluded researchers, but there are safe vaccines to protect you from viral infections and diseases. For Fungi Medicines applied directly to the infected area are available by prescription and over the counter for treating skin and nail fungal infections. Unfortunately, many people have had limited success with them. During the 1990s, oral prescription medicines became available for treating fungal infections of the skin and nails. For many years, very powerful oral antifungal medicines were used only to treat systemic (within the body) fungal infections, such as histoplasmosis. Doctors usually prescribe oral antifungal medications cautiously because all of them, even the milder ones for skin and nail fungi, can have very serious side effects. For Protozoa Diseases caused by protozoan parasites are among the leading causes of death and disease in tropical and subtropical regions of the world. Developing countries within these areas contain three-quarters of the world's population, and their populations suffer the most from these diseases. Controlling parasitic diseases is a problem because there are no vaccines for any of them. In many cases, controlling the insects that transmit these diseases is difficult because of pesticide resistance, concerns regarding environmental damage, and lack of adequate public health systems to apply existing insect-control methods. Thus, control of these diseases relies heavily on the availability of medicines. Doctors usually use antiparasitic medicines to treat protozoal infections. Unfortunately, there are very few medicines that fight protozoal infections, and some of those are either harmful to humans or are becoming ineffective. The fight against the protozoan Plasmodium falciparum, the cause of the most deadly form of malaria, is a good example. This protozoan has become resistant to most of the medicines currently available to destroy it. A major focus of malaria research is on developing a vaccine to prevent people from getting the disease. In the meantime, many worldwide programs hope to eventually control malaria by keeping people from contact with infected mosquitoes or from getting infected if contact can't be avoided.
Reference: National Institute of Allergy and Infectious Diseases, USA.
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Microbes belonging to the bacteria group are made up of only
one cell. Bacteria inhabit a variety of environments. Psychrophiles, or
cold-loving bacteria, can live in the subfreezing temperature of the Arctic.
Thermophiles are heat-loving bacteria that can live in extreme heat, such as
in the hot springs in Yellowstone National Park. Extreme thermophiles, or
hyperthermophiles, thrive at 235 degrees Fahrenheit near volcanic vents on the
ocean floor. Many bacteria prefer the milder temperature of the healthy human
body.
Viruses are among the smallest microbes, much smaller even than bacteria.
Viruses are not cells. They consist of one or more molecules
of DNA or RNA, which contain the virus's genes
surrounded by a protein coat. Viruses can be rod-shaped, sphere-shaped, or
multisided. Some look like tadpoles.
A fungus is actually a primitive vegetable. Fungi can be found in air, in
soil, on plants, and in water. Thousands, perhaps millions, of different types
of fungi exist on Earth. The most familiar ones to us are mushrooms, yeast,
mold, and mildew. Some live in the human body, usually without causing
illness. In fact, only about half of all types of fungi cause disease in
humans. Those conditions are called mycoses.
Protozoa are a group of microscopic one-celled animals. Protozoa can be parasites
or predators. In humans, protozoa usually cause disease. Some protozoa, like
plankton, live in water environments and serve as food for marine animals,
such as some species of whales. Protozoa also can be found on land in decaying
matter and in soil, but they must have a moist environment to survive.
Termites wouldn't be able to do such a good job of digesting wood without
these microorganisms in their guts.
