HIV is a virus and a virus has to infect a host before the virus can exert its effect. So, the first question is how do we get the virus - transmission. The simple answer if blood exchange. But it is too simple. T-helper lymphocytes and macropahges in particular that are of interest because these two cell types act as viral factories so that you get more than a virus particle which has to wander around, find the right cell, get inside, and replicate.
Cells are really bad news.
So, if you get infected cells it is worse than getting virus particles, but still enough particles will equal an infected cell. Notice that there are no simple hard and fast rules here. I could simplify and throw out information so that only a sound bite is left, but then a knowledge-based decision is compromised. Where are the portals of viral export and entry. How where does it get out of one established host and into the next new host.