Superinfection

Nature 
Superinfection is infection of an already infected person by a more virulent strain of the infective agent, such as a virus. It counters the general principle in medicine that infection protects against infection -- that if a person with an infective disease, they is protected from contracting a worse form of that disease.
Incidence 
Superinfection has been documented for HIV in humans. It is likely to reduce the effectiveness of an eventual vaccine against AIDS.
Broader 
Type 
(E) Emanations of other problems