In the 90s, a study of hetero-sexual couples where one partner was HIV positive found that transmission occurred 50% annually when coitus frequency was between 150 and 300 times per year. Transmission during heterosexual intercourse occurs, but infrequently.

Anal intercourse and active STD infections increased the frequency of HIV transmission.

There continues to be significant debate about the frequency (even possibility) of HIV transmission as a result of oral sex. Few, if any, fully documented cases exist. STD infections from oral sex DO occur, though infrequently.

Even if a sex worker has a 'higher load' of HIV cellular activity, unprotected sex of one or two incidences is statistically unlikely to transmit the virus.

On the other hand, shit does happen.

Now, an opportunity to share my favorite HIV joke:
In Harlem, one drug addict asks his friend if he can share his 'works' to inject. His friend says: "You want to use my needle? Aren't you afraid of getting HIV?" The first drug addict smiles slyly, "No, I'm not worried. I'm wearing a condom."