https://www.ncbi.nlm.nih.gov/books/NBK19466/Consider retroviruses, the viral category HIV/AIDs belongs to. It enters the human body as a viral particle, and
encodes itself onto the hosts DNA. That means the host now produces HIV viruses as a part of its own DNA. The only way to cure HIV/AIDS is to nuke all the tissue which produces HIV/AIDS viruses with extreme chemotherapy, then replace it with donor tissue.
https://www.nbcnews.com/health/health-news/7th-person-hiv-cured-stem-cell-transplant-leukemia-scientists-say-rcna161897Since a retroviruses' only incentive is to replicate itself, with making the host sick an unintended consequence which can negatively affect the retroviral life cycle, retroviruses have a strong incentive to either not hurt the host or to actually contribute to its life cycle success (chunks of the human genome are thought to have originated as retroviruses.
In order for scientists to find an STD, they first need to find an illness, then apply for funding, then test all those with that illness against control populations using various molecular biological techniques and diferentially compare the two populations. That's how HIV/AIDS was discovered, and it's all very expensive, and very complicated.
Long story short, there are thousands of undiscovered viruses, bacteria and funguses that are being passed around.
