I did survey research in my college years. The HIV survey cited above is one of the most flawed I've ever seen conducted. It was NOT a random sample.
$30.00 was paid to each participant. The result was most of the participants were IV drug users. As everyone should know the HIV rate is much, much higher in "needle users" than the population at large.
The best survey I know regarding HIV in TJ was conducted a few years ago. They tested all the women (expect about 7 who refused) who gave birth between xxx and yyy at Tijuana General for HIV. Of the over 900 tested the rate was about 1%, the same as in population at large of New York City.
Tijuana General is where poor women without insurance give birth. It is where those who work in the Zona Norte usually give birth. So while due to socioeconomic factors it wasn't a truly random sample of Tijuana it is the best indication I've seen of the HIV rate among Zona Norte "putas."
When looking at surveys one should look at the methodology and who is funding the survey. The purpose of many of these surveys is to get/maintain HIV/AIDS funding. In most cases there is a benefit in obtaining high HIV rate results.
Finally, this doesn't mean it is safe to go bare back. There is always a risk--the only question is how much risk.