Um i'm not 100% sure but i'm pretty sure you don't need to know the stages of HIV. So far in the past papers that i've done the only thing they can mainly ask you is how does the infectious disease, HIV, spread from one to another and, recently i came across this question, why is vaccination of this disease proved to be problematic. So as far as i know you don't need to know about the stages of HIV. 
Problems with vaccination:
Viruses are constantly mutating and changing - the reason we cannot create a vaccine against the common cold or influenza- by the time we have, it'll have mutated and the vaccine will not be effective. These mutations are known as antigenic shift or drift. Also, diseases like malaria are eukaroytic in nature and have far more genes and thus antigens on their cell surfaces.
Also, people sometimes do not respond well to vaccinations, maybe because their immune system cannot handle it, or they do not have enough protein to make antibodies - and thus the vaccination attempt may infect them. Also, people infected with a live virus may pass it out in their faeces during the primary response, potentially infecting others. This is why we vaccinate everyone at the same time, known as herd immunity.
Some viruses evade attack by the immune system by living inside cells - plasmodium (causes malaria) enters the liver and red blood cells, protected against antibodies in the plasma. Some parasites cover their bodies in host proteins, so that the immune system cannot see them. Vaccines cannot be easily produced against these because there is a very short period of time for the immune response to occur before the pathogen hides.
