Hello from JustAnswer.
Oral sex is far lower risk than penile-vaginal sex or anal sex.
For most STDs, the risk is about 0.005% per encounter, if she was positive for the disease in the throat. For example, if she is HIV positive and is not being treated, then the average risk of transmission is 0.005%. If she is of unknown HIV status, then the overall risk would be lower by a factor that reflects the prevalence of HIV in your community, or within the prostitute portion of the population in your community.
Other common STDs can also be transmitted by oral sex, but their risk is even lower because these other STDs, such as gonorrhea and chlamydia do not grow well in the mouth or throat. So even if the woman is infected with gonorrhea or chlamydia, it is very unlikely that they would be infected in the mouth or throat or that the germ would be transmitted via oral sex.
The infection that requires special comment is herpes. Oral herpes can be transmitted to the genitals via oral sex if the partner is susceptible. If you have a history of oral herpes, even as a child, then you are immune and cannot be infected again. And the majority of people have been infected with oral herpes. But if you are one of the minority of people that have never had oral herpes, then you are susceptible, and you can become infected either orally or genitally. If the herpes is inactive, then transmission cannot occur, but if the herpes is active and shedding virus, then the virus can be transmitted. The absence of a lesion does not indicate safety, because the virus can sometimes be shed without an active lesion, The overall risk of acquiring herpes is about 0.02-0.05%. The fact that the virus is not being shed on most days is reflected in this number, but if you know that there was an active lesion, the risk would be higher.
The time to symptoms varies according to the infection. It can vary from symptoms occurring within a few days in the case of gonorrhea and chlamydia, to 4-7 days for herpes, and 2-4 weeks for HIV.
Treatment would vary according to the STD. Gonorrhea and chlamydia are readily treatable with antibiotics. There is no treatment to cure herpes or HIV, but there are medicines that can suppress the growth of these viruses.
If I can provide any additional information, please let me know.