Is there evidence that veneral diseases originally became infectious after human and animal intercourse?

While some sexually transmitted diseases (STDs) like chlamydia and gonorrhea are known to have originated from animal precursors, there's no direct evidence that venereal diseases initially became infectious solely due to human and animal intercourse. The evolutionary history of STDs is complex, involving multiple factors and species interactions.

Here's a brief summary of the origins and transmission of some common STDs:

1. Chlamydia: Chlamydia is a bacterial STD caused by *Chlamydia trachomatis*. It's believed that *C. trachomatis* evolved from a strain that caused a respiratory infection in animals. Over time, it adapted to infect the urogenital tract in humans, likely through close contact and sexual activity.

2. Gonorrhea: Gonorrhea is a bacterial STD caused by *Neisseria gonorrhea*. It's thought to have originated from a non-pathogenic strain of bacteria present in the throats and respiratory systems of humans and other primates. Through sexual contact, the bacterium evolved to colonize and cause infections in the urogenital tract.

3. Syphilis: Syphilis is caused by the bacterium *Treponema pallidum*. Its origins are less clear, but most theories suggest it emerged in the Americas prior to the Columbian Exchange. Some researchers propose that *T. pallidum* evolved from a non-venereal spirochete present in animals, while others believe it may have originated from a human pathogen that adapted to sexual transmission.

4. Herpes Simplex Virus (HSV): The herpes simplex virus type 1 (HSV-1) primarily causes oral herpes, while type 2 (HSV-2) is responsible for genital herpes. Both HSV-1 and HSV-2 are believed to have originated from a common ancestor that infected primates. Over time, they evolved to transmit through close contact, including sexual activity.

5. Human Immunodeficiency Virus (HIV): HIV is the virus that causes AIDS. It's believed to have originated from simian immunodeficiency virus (SIV) in primates, such as chimpanzees and monkeys, likely through cross-species transmission (zoonosis) to humans.

It's important to note that the exact mechanisms and evolutionary pathways of STD transmission and adaptation are complex and still being studied. While animal reservoirs may have played a role in the origins of certain STDs, their transmission and establishment in human populations are influenced by various factors such as sexual behavior, hygiene practices, and changes in host-pathogen dynamics over time.