Dengue fever
There are four different types of the dengue virus.  Once a person has had one type of the virus, he usually is protected from that type for the rest of his life.  However, he will only be protected against the other three types of the virus for a short time.  If he later gets one of those three types of the virus, he will be more likely to have serious problems, like dengue shock syndrome or dengue hemorrhagic fever.