The key is for certain bodily fluids to be exchanged between two people -- one of whom has HIV.
Major ways include:
- Unprotected sex (certain types are riskier than others). This includes M/M (probably highest risk), F/F and F/M (probably the greatest current source of spread).
- Blood transfusions in the early 80s (not now);
- IV drug abuse with shared needles;
- Accidental exposure of health care workers (needle stick, surgery nick)
- HIV positive moms to newborns.
Other ways that are, at least in theory, possible (if very rare or undocumented):
- contaminated medical instruments (there is thought to be one case in Florida with dental intruments that had not been properly sterilized)
- Any other fluid exchange. For example kissing is thought to be very low risk. OTOH, if both persons had open wounds inside the mouth...
- Blood to blood contact (i.e., boxing).