...And Bad MicrobesWith such a variety of microscopic organisms, it's bound to happen that some do not help anything in the world. Some also help the world. We cover those in another section. We're going to cover a few of the bad ones here.
DiseasesMany species of bacteria cause disease in humans, animals, and even plants. Humans worry about bacteria that cause botulism (bacteria living in spaces without oxygen, such as cans), tetanus and E. coli. You should know that there are also some good forms of E. Coli living in your intestines. They help break down food and live a simple life (and yes, they make it smell down there). There are also E. Coli that can be passed to you from undercooked meat. These bad bacteria can make you very sick and even kill you.
A Role in Natural SelectionWe don't know of any viruses that are good for the world. They are an important piece of evolution and natural selection. Weaker and older animals are more easily infected. Those organisms are removed from the population so that healthier animals can survive. But the virus life cycle, that of a parasite, only hurts the organisms. Some even destroy cells in order to reproduce. And don't think you are the only one to get sick. Viruses attack plants and even bacteria. No organism is safe from damage. Examples of viruses include Rabies, Pneumonia, and Meningitis.
Sorry About That (Again)Humans are actually creating stronger bacteria and viruses by accident. The idea of natural selection is that weaker organisms are killed off and stronger ones survive and duplicate. Think about a bacterium for a moment. If you take antibiotics that kill bacteria, you get better.
However, because of variety, some bacteria may survive your medicine. Not enough bacteria survive to hurt you now, but they are there. If they eventually get someone sick, there is a chance that the antibiotics will not work again. You have incubated super bacteria! It's happening all the time in hospitals. We are killing off the easy diseases but some mutant strains are surviving. We might not be able to cure people next time a bad disease infects people.
Helping Plants Defend Themselves (USDA Video)
Encyclopedia.com (Lyme Disease):
Encyclopædia Britannica (Infectious Disease):