Cast of Characters

Natural History, Feb, 1999 by Rachel Zoffness

A brief guide to agents of infection

Viruses

Viruses come in myriad symmetrical shapes but share a general structure: a core of RNA or DNA surrounded by a protein shell. With no metabolic functions of their own, viruses can survive for limited periods outside the body of the host. Only when they infiltrate living cells do they begin to replicate, usurping the cell's genetic machinery to produce a profusion of viral particles. These "progeny" may burst out of the infected cell or simply bud off the cell membrane, repeating the process until they either destroy the host or are overcome by the immune system.

For a single-celled host organism, such as a bacterium, viral infection is usually lethal (see "Do Viruses Control the Oceans?" page 48). Plants, too, are extremely susceptible: viruses rank second only to fungi as crop destroyers. In humans, viral illnesses run the gamut from inconsequential to major. At least thirty-five types of papilloma viruses are responsible for warts. Rhinoviruses produce the universal and incurable common cold. Measles, HIV/AIDS, influenza, dengue hemorrhagic fever, and hepatitis B--all caused by viruses--kill millions annually worldwide (see "The World's Top Ten Infectious Killers in 1997," page 46). Less widely known is the fact that 15 to 20 percent of cancers (including malignancies of the cervix and liver) are of viral origin (see "Catching On to What's Catching," page 34).

In response to a viral invasion, animals produce antibodies that can, if the host survives, confer immunity to subsequent infection by that virus. Vaccination with a weakened or noninfectious form of the virus also stimulates production of immunity-conferring antibodies.

Bacteria

Bits of cytoplasm surrounded by a membrane, bacteria are the most abundant living things on earth. These unicellular beings comprise two kingdoms--the archaebacteria, found in extreme environments (such as hot seafloor vents), and the eubacteria, ubiquitous in soil, water, and both on and in the bodies of animals. More so than plants, animals are appealing habitat for bacteria, providing the moisture, food, and substrates that these microbes require for survival. Millions harmlessly colonize our skin and orifices as well as our respiratory and intestinal tracts. Some gut bacteria are useful house guests, providing essential vitamins and keeping fungi and harmful bacteria in check by competing for space and nutrients.

Bacteria that are innocuous in one part of the body may cause problems elsewhere--as when intestinal bacteria escape into the bladder or peritoneal cavity. But most disease bacteria come from the outside--such as tuberculosis bacilli that are inhaled in moisture droplets. Some bacteria cannot survive in air; Clostridium tetani, for example, secretes its deadly nerve toxins only after gaining access to the body's internal tissues through punctures, burns, and surgical wounds. Pathogenic Escherichia coli (a different strain from the E. coli that normally inhabits our intestines) causes internal hemorrhaging when consumed in contaminated food.

Of the ten major types of infection killing humans today, five--acute lower respiratory infections, tuberculosis, diarrheal diseases, whooping cough, and tetanus--are caused by bacteria.

Fungi

Fungi--the yeasts, molds, smuts, mushrooms, and mildews--constitute their own kingdom in the natural world. As purveyors of human misery, they take a backseat to bacteria and viruses. Some live harmlessly in the dead cells of our nails, hair, and feet, but most are soil dwellers. The few that regard humans as habitat cause annoying maladies such as yeast infections, athlete's foot, and the misleadingly named ringworm. As for the rest, we (and other animals) manage to fend them off with naturally occurring substances in our skin, blood, sweat, and saliva.

A little over a decade ago, however, a key study signaled a change: researchers found that nearly 40 percent of patients dying from hospital-acquired infections were being felled by fungi. The reason was a good news/bad news tale. Most people with compromised defenses--those with AIDS, cancer, organ transplants, and severe burns (as well as premature babies)--were surviving longer because of improved treatments. But fungi such as Candida (a common inhabitant of our mouth and other orifices) were taking advantage of catheters, needles, and incisions to enter the tissues of these immune-suppressed patients.

Plants are more vulnerable than animals to infection by fungi. Wind-borne fungal spores have caused such well-known and devastating epidemics as Dutch elm disease and the potato blight that led to the great Irish famine of the 1840s. Lesser known but economically significant ailments of today include karnal bunt disease in wheat and root rot in trees.

Fungi, which compete with other microbes for niches in the ecosystem, produce chemical defenses to ward off their bacterial rivals--a conflict that benefited humans when fungal defense chemicals were used to create penicillin, the mighty antibiotic that cures bacterial diseases from pneumonia to syphilis.