Alright, what the Hell is Wrong with IDiots who think that evolution is 'just another theory'. Do you people just blatantly ignore evidence? Do you already come in with a false conclusion hammered into your brains?
Viruses and Bacteria easily mutate into different strains; does this knowledge that should be considered common sense somehow become compartmentalized in peoples heads? All organisms have a rate of mutation per base pair of DNA before they replicate or 'have sex'. The change in the frequency of alleles in the populations of the organisms can change depending on whether or not those mutations result in increases to fitness (ability to reach sexual maturity and reproduce) or decreases in fitness. Helpful mutations normally propagate, leading to changes in the frequency of the alleles in the population. Bacteria and viruses when subjected to medicines that kill them off, are subject to a form of artificial selection, resulting in the increase in the population of bacteria or viruses that hold genes that when expressed allow the organisms to survive. These changes in the genes are due to random mutation and if the organism with the helpful genes is able to reproduce, then the genes begin to propagate in the population. If the random mutation in the population does not produce the genes that protect the organism from medicines or other harmful effects that organism goes extinct.
^^^^^this is Evolution^^^^^^^