POSTED 1 NOV 2000
The sickliest crew
Infectious diseases are caused by pathogens -- viruses, bacteria, parasites or
fungi. Before we talk about the origins of infectious disease, let's get
acquainted with this deadly gang.
Viruses:
Take a gob of genetic material -- either DNA or RNA -- and wrap it in some
proteins. Call those proteins a capsid, and you've got a weird half-living
thing called a virus. Tiny, ubiquitous, and deadly, viruses are dead -- until
they latch onto their preferred type of cell. From that point, they embark on
a single-minded mission to make more copies of themselves. Like a gang of
refuseniks who have commandeered a Soviet copy machine, they put the machinery
to their own purposes (only instead of copying subversive newsletters, the
cell cranks out more viruses). 
Often, the host cell dies in the process of
making so much virus. Viruses cause nasty diseases like AIDS, encephalitis,
yellow fever, hepatitis, herpes and polio. An especially grim group, the
filamentous viruses, includes the deadly Ebola and Marburg viruses.
Marburg virus between two human liver cells, magnified 75,000 times. Related to Ebola, Marburg also causes deadly hemorrhage.Electron micrograph by F.A. Murphy, University of California at Davis.
But don't feel discriminated against just because you're a primate. Other viruses are designed to attack plants, fungi, even bacteria. (More on viruses.)
Bacteria:
Bacteria are single-celled organisms that occupy their own branch of the tree
of life -- actually two branches, if you insist on accuracy. Generally,
bacterial cells are contained by a rigid cell wall and a rubbery membrane.
Inside, the DNA is loose rather than contained within a nuclear membrane, as
it is in all other forms of life. Like multi-cellular organisms, bacteria have
ribosomes -- structures used to multiply DNA. On the outside -- kinda like
tail fins on a '59 Caddie -- you might see a threadlike flagella that some
bacteria use to move about.
Bacteria are much larger than viruses, and like any adult, have more responsibilities. In fact, most bacteria have everything they need to make identical copies -- called "daughter cells." Viruses, in contrast, must latch onto a "host" cell before making copies of themselves. Bacterial infections include sweethearts like Staphylococci, Streptococci, anthrax, botulism and cholera. But bacteria are not all nasty. Your intestines are crammed with E. coli bacteria, which help you digest your food, and bacteria help in many ecological processes. More on disease bacteria.
Parasites:
Like viruses and bacteria, parasites live at the expense of other organisms.
However, a membrane around the cell nucleus indicates that they are more
evolutionarily developed than bacteria.
Giardia lamblia, a single-celled parasite, is anything but gentle on your guts.Courtesy Center
for Disease Control Division of Parasitic Diseases
Parasites can have one or many cells. Single-celled parasites include malaria, the deadliest infection on Earth, which kills two to three million people each year. Like many viruses, the parasite is transmitted by mosquito bites. (Malaria, an age-old disease that attacks red blood cells, is considered "emerging" because malarial mosquitoes now resist pesticides that once killed them, and because the parasite itself now resists many once-effective drugs.) Other single-celled parasites include Giardia and amebic dysentery, both found in foul drinking water, and both causing severe diarrhea. Multi-cellular parasites include trichinosis, hookworm, roundworm and tapeworm. (Ravenous for a write-up on parasites?)
Fungi:
Often found in soil, wood or decaying vegetation, fungi are actually closer in
evolutionary terms to us than to bacteria. Fungi are decay organisms that can
digest good stuff like plant matter and garbage. They are grouped as yeasts
(single-cell organisms) and molds (with multiple cells, they grow in the
characteristic filamented style you've seen on green bagels).
The histoplasmosis fungus infects lungs, particularly in people with inadequate immune systems. Courtesy Medical Mycology Research Center, Department of Pathology, University of Texas Medical Branch
Histoplasmosis, a fungal infection of the lungs and gastrointestinal tract, is a particular problem among AIDS patients and others with compromised immune systems.
If they're so tough, why haven't the bugs won?
