What's the probability to be infected with preseminal fluid? High? Low? I recently had an ocasional relation without a condom at the beginning but with it after and without ejaculation. I don't know if he's positive (he affirms he's not but...). Thanks a lot.
Preseminal fluid contains HIV and is indeed infectious. Look at the following article which has lots of information on the issue:
Also, unless you are in a long-term, mutually monogamous relationship in which both partners have tested negative for HIV, never take anyone's word that about their serostatus. Always practice safer sex in case the other person is positive. Remember, over 90% of people living with HIV do not know it because they have never been tested for HIV.