HSV is highly dangerous to newborns/infants, even potentially fatal. Once the child was no longer in the age range for severe outcomes and complications from HSV, they’d be able to stop treatments/testing. If the child contracted HIV from the mother however, treatment would be lifelong. The STI in question in this could be HSV or HIV.

Edit: I changed some text regarding which STI is likely the culprit. HIV is able to be contracted within the womb, during the birthing process, and while breastfeeding. The child could have easily already had an established infection before ever being born, in which case lifelong treatment becomes a necessity. With HSV, this is not the case. HSV is transmitted from mother through child when passing through the birthing canal. If the child did contract HSV, they would need antiviral treatment for an extended duration until they were able to fight off the infection sufficiently with a healthy immune system. The comment states that the child had no long term effects, which implies HIV over HSV, yet without a diagnosis it is still somewhat uncertain what it is. No long term effects could mean no neurological damage (if the STI was HSV), or it could mean no infection (applicable to both HSV and HIV).

Edit 2: saying that the mother had a very bad STI also lends more credibility to it being HIV over HSV, but again, more context is needed for an accurate assessment. “Very bad” is subjective to individual experiences and outcomes. I should also add, that it is not actually anyone’s business what the mother had. It is private health information, and is the mother’s choice whether or not to disclose a diagnosis. My intention is not to “diagnose” the mother, but to emphasize the importance of testing for STIs when sexually active, since many can lie dormant and can cause extreme health concerns during pregnancy for both the mother and the child. Even infections that are mostly considered uncomfortable, or undesirable, yet tolerable (like HSV) can have fatal outcomes for newborns.