Microbes in Sickness and in Health - Publications, National Institute of Allergy and Infectious Diseases: NIAID


Article title: Microbes in Sickness and in Health - Publications, National Institute of Allergy and Infectious Diseases: NIAID
Conditions: Microbes, B-cell, antibodies, DNA, immune, RNA, T-cell, Bacteria, Viruses, Fungi, Protozoa, smallpox, malaria, polio, bubonic plague, plague, influenza, Cryptosporidia, Giardia, Salmonella, Acute infections, Chronic infections, Latent infections, Shingles, Chickenpox, Protozoan, West nile virus, Hepatitis C, hantavirus, hantavirus pulmonary syndrome, TB, Malaria, Flu
Source: NIAID

MICROBES in Sickness and in Health


PDF Version

What Are Microbes?

Microbes are tiny organisms - too tiny to see without a microscope, yet they are abundant on Earth. They live everywhere - in air, soil, rock, and water. Some of them live happily in searing heat, and others in freezing cold. Like humans, some microbes need oxygen to live, but others cannot exist with it. These microscopic organisms are in plants, animals, and in the human body.

Some microbes cause disease in humans, plants, and animals. Others are essential for a healthy life, and we could not exist without them. Indeed, the relationship between microbes and humans is very delicate and complex. In this booklet, we will learn that some microbes keep us healthy while others can make us sick.

Most microbes belong to one of four major groups: bacteria, viruses, fungi, or protozoa. A familiar, often-used word for microbes that cause disease is "germs." Some people refer to disease-causing microbes as "bugs." "I've got the flu bug," for example, is a phrase you may hear during the wintertime to describe an influenza virus infection.

Since the 19th century, we have known microbes cause infectious diseases. Near the end of the 20th century, researchers began to learn that microbes also contribute to many chronic diseases and conditions. Mounting scientific evidence strongly links them to some forms of cancer, coronary artery disease, diabetes, multiple sclerosis, autism, and chronic lung diseases.

Top of Page

Note: Words in bold are defined in the glossary at the end of this booklet.


Microbes belonging to the bacteria group are made up of only one cell. Under a microscope, bacteria look like balls, rods, or spirals. Bacteria are so small that a line of 1,000 could fit across the eraser of a pencil. Life in any form on Earth could not exist without these tiny cells.

Scientists have discovered fossilized remains of bacteria that date back more than 3.5 billion years, placing them among the oldest living things on Earth. 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.

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.

Some bacteria produce poisons called toxins, which also can make us sick.

Streptococci Bacteria


Certain bacteria give off toxins that can seriously affect your 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.

Top of Page


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.

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).


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.

Mycoses can affect your 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.

Top of Page



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.

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.


  MICROBES IN THE HEALTHY HUMAN BODY* Microbe found in: Ear (outer) Aspergillus (fungus) Skin Candida (fungus) Small intestine Clostridium Intestines Escherichia coli Vagina Gardnerella vaginalis Stomach Lactobacillus Urethra Mycobacterium Nose Staphylococcus aureus Eye Staphylococcus epidermis Mouth Streptococcus salivarius Large intestine Trichomonas hominis (protozoa) *A selection of usually harmless microbes, some of which help keep our bodies functioning normally. If their numbers become unbalanced, however, these microbes may make us sick. All are bacteria, unless otherwise noted.

Top of Page

Microbes Have Bothered Us for Millennia

Microbes have probably always caused diseases in humans. Since ancient times, historians have documented some of those diseases, and present-day archeologists and microbiologists are discovering evidence of infectious disease in prehistoric human skeletons.

In a fascinating find in the late 20th century, researchers uncovered evidence in the mountains of northern Italy that prehistoric humans were troubled by microbial parasites and used natural remedies against them. Along with the frozen mummy of the "Ice Man", who lived between 3300 and 3100 B.C., scientists found a type of tree fungus containing oils that are toxic to intestinal parasites. Later in the laboratory, researchers found the eggs of a microscopic parasitic intestinal roundworm, Trichuria trichiura (whipworm), in his intestines.

Smallpox, which is caused by a variola virus, was described in ancient Egyptian and Chinese writings. According to some researchers, over the centuries smallpox was responsible for more deaths than all other infectious diseases combined. It killed millions of people over thousands of years before being eradicated late in the 20th century by worldwide vaccination. The last case of smallpox was recorded in 1977.

The protozoan parasite Plasmodium causes malaria, a tropical disease that usually is transmitted to humans during the bite of the Anopheles mosquito. In ancient times, this disease was mentioned in Egyptian writings called hieroglyphics and was described in detail by the Greek physician Hippocrates. Malaria ravaged invaders from the Roman Empire. Though rare in the United States, malaria remains a serious public health threat worldwide. It kills 3 million people each year, most of whom are children.

Evidence on a 1300 B.C. Egyptian stone engraving shows that poliomyelitis (polio) has been around since ancient times. In the 1990s, public health officials launched a massive international vaccination campaign to eradicate the polio virus, which causes paralysis and can be deadly. Polio has been virtually eliminated in the United States and in much of the rest of the world.

In the 14th century, a bacterium scientists later identified as Yersinia pestis caused the bubonic plague, or Black Death. Bubonic plague entered Europe and Africa through infected rodents and fleas that accompanied travelers along trade routes from Mongolia. The plague epidemic spread through Europe, Africa, and the Middle East, killing about 20 million people in Europe alone. Plague is spread to humans through the bites of fleas, which pick up the bacteria while sucking blood from rodents, especially rats. In the United States, health care workers report cases of plague even today, most of which are found in the Southwest.

Viruses caused two major pandemics during the 20th century. From 1918 to 1919, the influenza virus ravaged worldwide populations. Estimates of the number of people killed during the so-called "Spanish flu" pandemic range from 20 million to 40 million. HIV, which was identified in 1984, had killed 36.1 million people worldwide by the end of 2000.

Top of Page


Date: Event: Approximately 300 B.C. Aristotle, Greek philosopher and scientist, studied and wrote about living organisms. 1675 Antony van Leeuwenhoek discovered bacteria. 1796 Edward Jenner laid the foundation for developing vaccines. 1848 Ignác Fülöp Semmelweis discovered simple handwashing could prevent passage of infection from one patient to another. 1857 Louis Pasteur introduced the germ theory of disease. 1867 Joseph Lister showed evidence that microbes caused disease and pioneered the use of antiseptics during surgery to kill germs. 1876 Robert Koch, by studying anthrax, showed the role of bacteria in disease. 1928 Alexander Fleming is credited with discovering penicillin.

Microbes Can Make Us Sick

According to health care experts, infectious diseases caused by microbes are responsible for more deaths worldwide than any other single cause. They estimate the annual cost of medical care for treating infectious diseases in the United States alone is about $120 billion.

The science of microbiology explores how microbes work and how to control them, and it seeks ways to use that knowledge to prevent and treat the diseases microbes cause. The 20th century saw an extraordinary increase in knowledge about microbes. Microbiologists (scientists who study microbes) and other researchers scored many successes in learning how microbes cause certain infectious diseases and how to combat those microbes.



Medical Tools & Articles:

Next articles:

Medical Articles:
CureResearch.comTM Copyright © 2010 Health Grades, Inc. All rights reserved.
Home | Contents | Search | Site Map | Feedback | Contact Us | Terms of Use | Privacy Policy | About Us | Advertise