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.