This Week’s Question: I remember having lots of fevers as a kid, but, now that I’m older, I don’t get them like I used to. What gives?

The immune system doesn’t function as efficiently in older adults as it does in younger people. The body's fever response to infection is not always automatic in elderly people. More than 20 percent of adults over age 65 who have serious bacterial infections do not have fevers.

This brings us to germs, which are defined as microbes that cause disease. Infectious diseases caused by microbes are the leading cause of death.

Microbes are microscopic organisms that are everywhere. Some microbes cause disease. Others are essential for health. Most microbes belong to one of four major groups: bacteria, viruses, fungi, or protozoa.

Bacteria are made up of only one cell. Less than 1 percent of them cause diseases in humans. Harmless bacteria live in human intestines, where they help to digest food. Foods such as yogurt and cheese, are made using bacteria.

Some bacteria produce dangerous poisons. Botulism, a severe form of food poisoning, is caused by toxins from bacteria. However, several vaccines are made from bacterial toxins.

Viruses are among the smallest microbes.They consist of one or more molecules that contain the virus's genes surrounded by a protein coat. Most viruses cause disease. They invade normal cells then multiply.

There are millions of types of fungi. The most familiar ones 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. Penicillin and other antibiotics, which kill harmful bacteria in our bodies, are made from fungi.

Protozoa are a group of microscopic one-celled animals. In humans, protozoa usually cause disease. Some protozoa, like plankton, are food for marine animals. Malaria is caused by a protozoan parasite.

You can get infected by germs from other people in many different ways, including transmission through the air from coughing or sneezing, direct contact such as kissing or sexual intercourse, and touching infectious material on a doorknob, telephone, automated teller machine or a diaper.

A variety of germs come from household pets. Dog and cat saliva can contain any of more than 100 different germs that can make you sick.

Mosquitoes may be the most common insect carriers of disease. Mosquitoes can transmit malaria. Fleas that pick up bacteria from rodents can then transmit plague to humans. The tiny deer tick can infect humans with Lyme disease.

We become immune to germs naturally and artificially. Before birth, we received natural immunity from our mothers. Once we are exposed to a germ, we develop natural immunity to it from special cells in our immune systems. Artificial immunity can come from vaccines.

Most infections caused by microbes fall into three major groups: acute infections, chronic infections and latent infections. The common cold is an acute infection. Hepatitis C, which affects the liver, is a chronic viral infection. Chickenpox is an example of a latent infection that can emerge many years later and causes a disease called “shingles.”

Handwashing is a simple and effective way to stop the transmission of germs. Health care experts recommend scrubbing your hands vigorously for at least 15 seconds with soap and water.

It is especially important to wash your hands before touching food, after coughing or sneezing, after changing a diaper, and after using the toilet.

The Healthy Geezer column publishes each Monday on LiveScience. If you would like to ask a question, please write © 2010 by Fred Cicetti.