Dengue fever is an infectious disease of humans that is caused by a virus. There are four main viral strains, and an individual can never be infected by the same strain twice. However, a second infection can occur by any of the other strains. The severity of the second infection depends on the identity of both the strain that caused the first infection and the strain causing the second infection, as well as the order in which these were acquired. Required:
How many different levels of severity are possible for an individual acquiring a second infection