Antibiotic resistant bacteria
The best-known is the scourge of medical facilities, methicillin-resistant Staphylococcus aureus, usually referred to as "MRSA".
Human ignorance plays a large role in increasing the prevalence of antibiotic-resistant bacteria. Contributing factors include:
- Not taking antibiotics for the prescribed amount of time.
- Taking antibiotics when one is not suffering from a bacterial infection.
- Using antibiotics in the production of animals raised for food
These sorts of antibiotic misuse will kill some of the bacterial population, but leave the bacteria that are more resistant to that antibiotic. These bacteria are then free to multiply and pass along their resistant genes. Future mutations in this population may (and do) eventually produce bacteria that are not affected at all by the original antibiotic. When bacteria have reached a level where they are immune to multiple antibiotics, they are often termed "super-bugs" by the media and much hype is given to them.
Often, the best way to prevent antibiotic resistance is to combine different drugs in a course of treatment. The odds of a strain of bacteria randomly mutating an immunity to one drug is low, but plausible, considering the millions of cells in a single infection and the millions of infections that may occur over time. Using multiple drugs simultaneously would require that strain to have developed multiple resistances. The odds of such a thing happening are dramatically lower than the possibility of a strain developing resistance to a single drug, and a bug that can survive all these antibacterial treatments may become less effective against the immune system or develop traits otherwise detrimental to its survival in other ways.
Bacteria can also be killed by less subtle means than antibiotics. Sterilizing areas with strong poisons such as alcohol, highly damaging substances such as bleach, or irradiation, will still work to destroy bacteria on surfaces. Although these methods are not suitable for treatment of an infection, they can reduce the number of infections. The reason that bacteria don't develop immunity to these things is similar to why they don't easily develop immunity to combined drug courses; the number of metabolic pathways that must be altered in a single mutated generation is much higher, and the odds of it happening are close to none, even with the vast number of bacteria in play. Developing a resistance to an antibiotic - typically, a strain of fungus - is like learning to outsmart a predator. Developing a "resistance" to bleach is like learning to breathe on the Moon.
 Evolutionary interest
Antibiotic resistant bacteria are also one of the saddest, but strongest, examples showing the process of evolution, and why it should be taught properly. Despite the differences between how bacteria and animals evolve, the premise of natural selection is clearly shown to work in this example. Some people, however, don't accept this as evidence of evolution as the bacteria remain the same "kind", i.e., they haven't evolved into a horse.
The reason why bacteria are able to genetically "respond" to human selective pressures so fast, and in a human observable period of time, is by virtue of them having many generations in a relatively small period of time. (There are bacteria that can double their population every 10 minutes). This is also the case with viruses which are capable of millions of generations before the end of the week.