British beef blues

Curious cause

Down deer, ill elk

Can't happen here?

Laughing death in New Guinea

Identifying disease agents

Menacing microbes

Glossary

But does it make 'em sick?
Like all supposed causes of disease, the notion that aberrant prions cause BSE (Want The Why Files guide to mad-cow lingo?) and related diseases -- must satisfy the "Koch postulates." The pioneering German microbiologist Robert Koch argued -- and scientists now accept -- that only after a positive answer to four questions can we say that agent "X" causes disease "Y":

The agent must be present in every case of the disease;

The agent must be isolated from the host and grown in a lab dish;

The disease must be reproduced when a pure culture of the agent is inoculated into a healthy susceptible host; and

The same agent must be recovered again from the experimentally infected host.

So it's cut and dried?
Not exactly. The hallowed postulates, a prime mover in the field of medical microbiology, have some limitations:

Some agents (including prions and viruses), do not grow in a lab dish, but only in a living cell.

Ethically speaking, you can't do your "healthy susceptible host" testing with people, but with lab animals or livestock. Testing the infectivity of a possible human pathogen in other animals always raises a question: If the pathogen doesn't infect the lab animal, does that mean it cannot infect humans?

In performing tests to satisfy Koch, careful scientists always use uninoculated control animals, so the only difference between the "experimental animals" and "control animals" is the inoculation, or deliberate infection. Controls are used to remove the chance that the experimental animals got sick for unrelated reasons, such as their genetic makeup or some experimental conditions.

Tell me more about microbes in their mysterious variety.