This has everything to do with the transmission route of a virus. To spread itself, a virus needs to travel from one animal or person to the next. Each type of virus has its own way of travelling: a transmission route. Flu travels through the air for instance, making it very easy to spread. When you’ve got the flu and you cough, you spread the virus all around you. HIV on the other hand is transmitted through blood, so you need to be in very specific conditions to get it. Through sexual interaction or an infected needle for example. Dengue or Malaria can only be transmitted by specific types of mosquitoes, which are mainly found in the tropics.