When evolving, microorganisms such as harmful bacteria may develop a resistance to antibiotics. This means that the disease the bacteria in question cause will be harder to treat. This is quite bad since bacteria can pass on traits to other bacteria without even multiplying.
The flu virus is also a good example. Generally, the virus itself mutates quite frequently. This means that with each mutation the virus has a potential to gain a genetic trait which may cause more severe flu symptoms or a resistance to drugs/the antibodies our bodies produced to counter-attack the type of flu virus it has previously encountered.
