If bacteria and viruses are mutating, how do scientists know what to put in the medicine if its forever changing?

Question: If bacteria and viruses are mutating, how do scientists know what to put in the medicine if its forever changing?
- Keywords:
  bacteria,
  drug,
  medicine,
  mutation,
  virus
Asked by 782yttc52 to Anna, Hayley, Iain, Rebecca on 15 Jun 2015.
- anon answered on 15 Jun 2015:
  
  That’s why science is so important – scientist are constantly researching how bacteria and viruses are mutating and they are always modifying the medication and treatments, and developing new ones.
- Anna Kutner answered on 15 Jun 2015:
  
  I’m not a microbiologists but I assume that certain mutations can be predicted if you have enough knowledge about the virus. When developing a vaccine you would use your current strain as a blueprint so that it can be at least partially effective even if the strain mutates.

Question: If bacteria and viruses are mutating, how do scientists know what to put in the medicine if its forever changing?