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A major theme of biology is evolution, and there is no better place to study it than with populations of bacteria or viruses (even better than studying it on a tropical beach). The quick replication of these populations allows the process of natural selection to be played in fast-forward. A new strain of virus can evolve very quickly if the right situation arises.
Let's think about a population of viruses where mutations are constantly occurring. A single mutation can affect how well that virus will survive to replicate. Some mutations are favorable, some are unfavorable, and some have no effect.
Those with unfavorable mutations will not replicate as efficiently. Therefore, viruses with an unfavorable mutation will eventually die out. On the other hand, those with favorable mutations will replicate more efficiently. They will have more progeny than the original virus and soon viruses with this mutation will become a larger and larger portion of the population.
A favorable mutation will take over a population until there are none of the original members left? Right. But what kind of a mutation would be favorable? Well, basically anything that helps a virus to replicate better—especially mutations that help them survive an antiviral attack.
Imagine you are a flu virus. You have a genetic condition called weirdo-shape, which makes you look different from the other viruses. You are at a party inside some guy's lungs. You and the other viruses are hanging out, going into cells, and replicating. Suddenly, some drug particles show up uninvited. They begin to attach to all the viral surfaces to prevent everyone from replicating. Oh no.
But they can't attach to your weirdo shape. You have a funny shaped surface and the drug doesn't know what to do. You feel sorry for the other guests, but you decide to party-on without them. You keep on replicating and replicating and replicating. Pretty soon the whole party is filled with funny shaped viruses just like you. In the days and months and years to follow, your weird shaped descendants live happily ever after without worrying about drug particles again. The End.
This is what happens when a virus is resistant to a drug. All it takes is the right "weirdo-shape" mutation to occur and natural selection will do the rest. The quick evolution of viruses and bacteria means research labs are endlessly searching for new treatments to keep up with the times.
A major theme in Biology is unity and diversity. The domain of bacteria is the perfect way to demonstrate this theme. Although it is the simplest of the three domains of life, bacteria are very different and can cause about a bazillion different diseases.
All bacteria are unicellular. They all consist of a plasma membrane with a nucleic acid genome inside. They all reproduce and require energy. However, the similarities do not go much further than that. Bacteria are remarkably diverse for a tiny microorganism.
Think about how different a single-celled organism must be to cause such different problems as leprosy and peptic ulcer disease. They have limited ways of being different (while still remaining simple prokaryotes), but they take the cake for making the most out of the least.
Their differences make them capable of living in a variety of habitats. This means that they can infect many different places in the body. Let's review this earlier figure again to see the different bacteria that infect each place in the body.
Common sites of infection for different types of bacteria.
Bacteria can infect all over the body. Some bacteria cause disease in your gut if you eat them like E-coli or Salmonella. Others cause lung infection if you inhale them, like Tuberculosis. Others cause infection of the genitals, like gonorrhea. For a tiny little bugger unified by its basic characteristics, bacteria can certainly cause trouble all over the body.
Unified bacteria of the same species can cause different symptoms depending on where they infect. Look in the figure at some of the names that appear in more than one section. These bugs, like Strep and Staph, can survive in many environments and the infections they cause in each environment are different.
For example, if Strep infects your throat you might get a sore throat and maybe lose your ability to sing for a few days. But if it infects a paper cut on your hand…well you might have to wave goodbye to that hand with the other hand and invest in a nice hook. Similarly, staph (the bacteria of MRSA) can cause food poisoning, an eye infection, or a skin infection.