evolution and mutation of microrganisms against medicines?
A few throughts about this: Microorganisms have a short life span but they replicate very fast with a short generation time. So this create vast amounts of "offsprings" with some mutations naturally occurring. Due to this large numbers the chance of developing resistances is much higher than for slowly replicating species. 

This is especially true, when the selection pressure is high, as this is the case when you treat with drugs which usually kill these microbes efficiently. When people are not taking these drugs (antibiotics for example) according to the necessary scheme, the concentration of the drug in the body is lower than necessary to kill of all microorganisms. This leads to the survival of microorganisms which are a bit adapted to survive the drug and so on.

The real problem are people who are not following their treatment regime necessary and thus causing resistancies. A case for this is tuberculosis. Regarding taking medication: Always take them when necessary and leave them out when not. A simple cold for example can not be treated antibiotics since it is viral, the secondary middle ear infection usually can.