That is a very interesting question that has fascinated scientists for a long time. It is now known that some people can have unprotected sex with people infected with HIV , and not get infected. The are various reasons for this, but one of those is about the amount of viral load circulating in the body of the infected person. There are times when the viral load is low, a low infective stage. The duration of this stage various across person. We cannot know, simply by looking at the physique of a person, whether they are highly infectious or not, and that is why it is strongly recommended that you use all measures to protect yourself, and to be regularly tested. Chilling is a very good preventive measure, and if you are you engage in sex, to be responsible and use condoms consistently and correctly.