Most people have exaggerated notions about the risks of HIV-transmission. Table 1 illustrates the results for current scientific research into these risks. The first value (transmission probability = 0.001) means that the risks are 1 per thousand - half of people would get infected in fewer than 1000 sexual acts and the other half only after more than 1000 sexual acts.

Similarily, many people (even doctors) believe that the risks for Injection Drug Users is comparable to that for those receiving donations of contaminated blood. The full article from which this table is extracted is to be found here.
Table 1. Per-act probabilities of transmission
Study population Route of transmission Transmission probability
Heterosexual couples
United States
(Peterman et al. 1998. Wiley et al.1989)		 Penile→vaginal 0.001
United States
(Wiley et al. 1989; Padian et al. 1987)		 Penile→vaginal 0.0008 - 0.001
United States
(Fischl et al. 1987)		 Penile→vaginal
and
Vaginal→penile		 0.001
Europe
(Downs et al. 1996)		 Penile→vaginal
and
Vaginal→penile		 0.0005 - 0.001
Heterosexual men
Kenya
(Cameron et al. 1989)		 Vaginal→penile
from FSWs to men who also acquired a symptomatic STD		 0.1
Thailand Vaginal→penile
from FSWs to men		 0.036 - 0.06
Homosexual men
United States
(DeGruttola et al. 1989)		 Penile→Anal
receptive		 0.005 - 0.03
Injection drug users
United States
(Kaplan and Heimer 1992)		 Shared injection equipment 0.0067