Elsewhere I posted about Tijuana prostitution laws.
Prostitutes are supposed to have health cards. Some do and some don't. Getting a health card requires a chica to be tested for some, but not all, STDs. (For example, they don't test for herpes.) So a health card does not necessarily mean a chica does not have a STD.
The system isn't prefect but has a positive impact on prostitution in TJ. The rate of STDs among Tijuana prostitutes is MUCH less than in the US.
Of all the categories of sex workers the percentage of street girls with health cards is the highest. That's because when Reglamento does its checks they can't duck into the ladies room or out the back door like bar girls do.
The larger the club the more likely a bar girl working there is to have a health card. But again, some do and some don't.
It's true many escorts want to fly below the radar. However, it is more common than most think for those without health cards to get private checkups. (Also they are low mileage which helps make them safer than some think.)
Contrary to urban myth: You have to go to a lab set up by the city to get a health card and the doctors and lab techs working there don't take bribes. (Nor are the chicas allowed contact with the lab techs.) On top of that it would be cost prohibitive to pay a monthly bribe to keep a health card active.
Many chicas who can't get a health card (usually because they have a STD, but sometimes because they aren't Mexican citizens, etc.) work the streets as freelancers. They are known as walkers because unlike street girls they're never in the same place. I recommend avoiding them a high percentage have STDS, most are hard core drug users, and they sometimes work with the rateros by setting up tourists to be mugged.
The requirement to get a health card is part of the business code, not the criminal code. Seeing a prostitute who lacks a health card violates no law. (It is the prostitute, not the customer, that has to obtain the health card. It is similar to buying a taco from a taco cart without a license: the owner, but not the customer, is the one who can get in trouble.)
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Going bare back always--even if a chica has a current health card--puts a man at risk. Only you can decide how much risk you're willing to take when hobbying.