Epidemic! The Natural History of Disease.  At the San Diego Natural History Museum

The Natural History of Disease

microbes, including a bacterium, virus, protozoan, helminth, and fungus
Microbes clockwise from top: Shigella dysenteriae (a bacterium causing dysentery); T4 bacteriophage (a virus harmless to humans); Plasmodium falciparum (a protozoan causing malaria, trophozoite stage; Ascaris lumbricoides (an intestinal roundworm); Histoplasma capsulatum (spore of fungus that causes histoplasmosis, a lung infection)

The Story of Microbes

Microbes are the oldest form of life on Earth. Some types have existed for billions of years. These single-cell organisms are invisible to the eye, but they can be seen with microscopes. Microbes live in the water you drink, the food you eat, and the air you breathe. Most microbes are helpful and some even essential, like the billions of microbes swimming in your intestines to help digest food and create the essential vitamins our bodies need. Billions more live naturally in our skin, mouth, nose, teeth, throat, and urethra. In fact, 95% of all microbes are not harmful.

Where Do Microbes Live?

Humans, microbes, and other living creatures all share the environment and interact in ways that allow them to coexist. Microbes' mission in life is to reproduce and do whatever it takes to survive. They have the ability to evolve rapidly and can adapt to changing conditions, but where any particular microbe can live depends on its biological requirements. Some, like the hantavirus microbe, are limited to the habitat of the animals that carry them for part of their life cycle. Some have evolved to exist in more than one habitat: different flu viruses can survive in humans, birds, and other animals, and can even withstand drying out on an exposed surface. Others are far more specialized and can survive in only one type of environment. Deadly and widespread as it is, HIV cannot exist for long outside the human body, for example.

Some disease-causing microbes enter the human body and stay there for part or all of their life cycle. When pathogenic microbes spend part of their lives in insects or other animals before they move to the human body, they are called vector-borne agents. Some microbes that live in water are harmful if swallowed, or if they penetrate the skin. Soil microbes can enter the human body through a break in the skin or can be inhaled as dust.

How Did Microbes Become Agents of Disease?

Humankind's initial method for survival was to hunt and gather food. Later, people developed agricultural societies, which sustain much higher human population densities than the hunter-gathering lifestyle. Jared Diamond, in his book Guns, Germs, and Steel, traces the history of how people's advancement in civilization and agriculture has profoundly affected how germs became closely associated with infectious diseases.

Whereas hunters frequently are on the move, farmers are sedentary and live close to their own sewage, thus providing microbes a short path from one person's body into another's drinking water. Some farming populations make it even easier for their own fecal bacteria and worms to infect new victims, by gathering their feces and urine and spreading them as fertilizer on the fields where people work. Irrigation agriculture and fish farming provide ideal living conditions for the snails carrying shistosomiasis and for flukes that burrow through the skin as people wade through feces-laden water. Farmers also become surrounded by disease-transmitting rodents, attracted by the farmers' stored food. Forest clearings made by African farmers also provide ideal breeding habitats for malaria-transmitting mosquitoes.

People began to domesticate selected plants and animals, using them for food, healing, herding, transportation, or companionship. Diverse epidemic diseases of humans evolved in areas with many wild plant and animal species suitable for domestication, partly because the resulting crops and livestock helped feed dense societies in which epidemics could maintain themselves, and partly because the diseases evolved from germs of the domesticated animals themselves.

The major killers of humanity throughout our recent history--smallpox, flu, tuberculosis, malaria, plague, measles, and cholera--are infectious diseases that have evolved from diseases of animals, even though most of the microbes responsible for our own epidemic illnesses are paradoxically now almost confined to humans. Diseases have been the biggest killers of people. Until World War II, more victims died of war-borne microbes than of battle wounds.

If the rise of agriculture, proliferation of populations, and domestication of plants and animals were a bonanza for our microbes, the rise of cities was a greater one, as still more densely packed human populations festered under even worse sanitation conditions. Another bonanza was the development of world trade routes, which by Roman times effectively joined the populations of Europe, Asia, and North Africa into one giant breeding ground for microbes. Today, our jet planes have made even the longest intercontinental flights briefer than the duration of any human infectious disease. The explosive increase in world travel by Americans, and in immigration to the United States, is turning us into another melting pot--this time, of microbes that we previously dismissed as just causing exotic diseases in far-off countries.

The World of Microbes

There are five types of microbes: bacteria, viruses, protozoans, fungi, and helminths (worms).

1. Bacteria
bacteria: E.coli, Mycobacterium, Strepococcus - AMNH

The most abundant organisms on Earth, bacteria live almost everywhere: in the soil and water, in plants and animals. Whether they take the form of spheres, rods or spirals, bacteria consist of a single cell. Unlike the cells of animals and plants, bacterial cells lack a nucleus, but they can carry out all necessary life functions. Most bacteria are parasites, although a few manufacture their own food. Some of these parasites are very helpful -- they aid in many bodily functions including digestion, and help with other processes, such as decomposition of soil and changing of milk into cheese. Disease results, however, when bacteria multiply rapidly (each cell simply divides into two identical cells) and damage or kill human tissue, as in pneumonia and tuberculosis. Diseases can also produce toxins that damage or kill human tissue, as in food poisoning or cholera. Sometimes bacteria in the body are helpful for a while, and then something in the body or the bacteria changes, causing destruction in the host.
Bacteria pictured clockwise from top: E. coli 0157:H7 (causes food poisoning), Strepococcus pyrogenes (causes strep throat), Mycobacterium tuberculosis (causes tuberculosis)

2. Viruses
viruses: Adenovirus, HIV, Influenza A - AMNH

By far the smallest microbes, viruses can appear as spirals, 20-sided figures or even more complicated forms. They consist mainly of genetic material--DNA or RNA. They are not cells, however, and cannot carry out life functions on their own. Living inside the cells of other species, viruses use the host cells to grow and produce new viral particles. As they take over genetic material to reproduce themselves, the host cells often die. Found in all groups of living things, from bacteria and fungi to plants and animals, hundreds of the known viruses can cause many kinds of infections, chickenpox, measles, flu, colds, polio, and AIDS. Viruses cannot move by themselves and must be carried to cells by air currents and then by body fluids to the cells. Some viruses may lay dormant for years before becoming active, as with AIDS. Most diseases come from other species, for example: smallpox from dogs or cattle, hemorrhagic fevers from rodents and monkeys, tuberculosis from cattle and birds, common cold from horses, and AIDS from African monkeys.
Viruses pictured clockwise from top: Adenovirus (causes the common cold), Influenza A (causes the flu), and Hepadnavirus (causes hepatitis B)

3. Protozoa
protozoa: Giardia                                                                              intestinalis (causes diarrhea), Trypanosoma, P. falciparum - AMNH

Protozoa consist of a single cell that includes a nucleus. The cell also contains structures that carry out specific processes needed for life functions. A diverse and complex group, protozoa range through many shapes and sizes. They can be parasitic, needing to live within another organism, or free-living in moist habitats. The similarity of inner structures of protozoan and human cells makes it difficult to treat infections caused by protozoa. Drugs that may destroy the protozoan may also destroy human cells. Protozoan infections include amebic dysentery, malaria, and African sleeping sickness.
Protozoa pictured clockwise from top: Giardia intestinalis (causes diarrhea), Trypanosoma brucei (causes sleeping sickness), Plasmodium gametocyte (causes malaria)

4. Helminths
helminths: Ascaris 
lumbricoides, Schistosoma
mansoni - AMNH

Other microorganisms break down body tissues or absorb digested food. They can cause anything from skin infections to internal disorders that can lead to death. The group called helminths includes flukes, roundworms, and tapeworms; these are many-celled animals with developed organs. Among the numerous types, some are parasites--organisms that live in or on another species, usually harming the host species in the process. Because of their size, parasitic worms grow outside of cells and can reach an astronomical size of 30 feet in length.
Helminths pictured left to right: Ascaris lumbricoides (an intestinal roundworm), Schistosoma mansoni (a parasitic worm that lives in contaminated water and causes schistosomiasis or bilharzia)

5. Fungi
Fungi: Histoplasma
capsulatum, Penicillium notatum - AMNH

Fungi include yeasts (one-celled), and mushrooms and molds (multi-celled). Unlike plants, fungi do not make their own food. Some species of fungi get their nutrition by breaking down remains of dead plants or animals. Others are parasites. Examples of fungal infections include athlete's foot and ringworm.
Fungi pictured left to right: Histoplasma capsulatum (causes histoplasmosis, a lung infection), Penicillium notatum (produces the drug penicillin)

Microbes Evolve, Just as Human Do

Microbes have evolved diverse ways of spreading from one person to another, and from animals to people. Jared Diamond tells the following story of how microbes have evolved to survive, just as people have evolved over time to deal with diseases.

The most effortless way a germ could spread is by just waiting to be transmitted passively to the next victim. That's the strategy practiced by microbes that wait for one host to be eaten by the next host: e.g., salmonella bacteria, which humans contract by eating already infected eggs or meat; the worm responsible for trichinosis, which gets from pigs to people by waiting for humans to kill the pig and eat it without proper cooking; and the worm causing anisakiasis, with which sushi lovers occasionally infect themselves by consuming raw fish.

Most microbes don't wait for the old host to die and get eaten, but instead hitchhike in the saliva of an insect that bites the old host and flies off to find a new host. Mosquitoes, fleas, lice, or tsetse flies that spread malaria, plague, typhus, or sleeping sickness, respectively, may provide the free ride. Microbes that pass from a woman to her fetus, thereby infecting babies at birth, perpetuate the dirtiest of all tricks for passive carriage of diseases like syphilis, rubella, and AIDS.

Other germs take matters into their own hands. They modify the anatomy or habits of their host in such a way as to accelerate transmission. The open genital sores caused by venereal diseases like syphilis enlist a host's help in inoculating microbes into a body cavity of a new host. The skin lesions caused by smallpox similarly spread microbes by direct or indirect body contact.

More vigorous yet is the strategy practiced by the influenza, common cold, and pertussis (whopping cough) microbes, which induce the victim to cough or sneeze, thereby launching a cloud of microbes toward prospective new hosts. For the modification of a host's behavior, nothing matches the rabies virus, which not only gets into the saliva of an infected dog but also drives the dog into a frenzy of biting, thus infecting many new victims. But for physical effort on the microbe's own part, the prize still goes to worms such as hookworms and schistosomes, which actively burrow through a host's skin from the water or soil into which their larvae had been excreted in a previous victim's feces.

Thus, from the human point of view, genital sores, diarrhea, and coughing are "symptoms of disease." But from a germ's point of view, they're clever evolutionary strategies to spread the germ and ensure its survival. That's why it's in the germ's best interest to "make us sick." From the germ's perspective, illness is just an unintended by-product of host symptoms promoting efficient transmission of microbes.

This section is taken from the Epidemic Teacher's Guide ,
which has additional information and activity ideas that parents can do with their children.

Banner microbe: Adenovirus (virus causing common cold)
from the American Museum of Natural History Epidemic! exhibition

Epidemic! | Exhibits