About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.
They also can evolve more quickly than most other organisms due to a shorter generation time (just 20 minutes for E.coli). An example of this is resistance to antibiotics, which can be result from a single duration of medicine.
Another reason they are successful is because they have adjusted to living within humans by evolving ways to evade the immune system, by for example displaying structures on their surfaces which are similar to those of their host, so the immune system does not perceive it as a foreign object. There are also bacteria which, unlike animals, can live without oxygen (anaerobic), some can alternate between many different respiratory pathways, using different electron acceptors apart from oxygen, such as nitrogen, nitrite and nitrate, and even sulphate, which also allows some to live deep within the human gut.
There are also special appendages, such as fimbriae which allows attachment to the host surface and bacteria can form biofilms, which offers protection against the immune system and antibiotics. Bacteria can also produce antibiotics of their own to complete with its own and other species of bacteria.
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