STD/HIV can take years to appear ?
would you be more specific? STDs generally classified into 2 broad categories- bacterial and viral.
Bacterial infections, e.g. gonorrhea, syphilis, yeast infections, etc, usually have a shorter incubation period, the symptoms show up real quick. A good dose of anti-biotics and creams can help clear the symptoms.
Viral infections on the other hand, e.g HIV, the incubation period could take a while longer. Which means you're a carrier without showing any symptoms, in short- you're a walking time-bomb stuffing dynamites into the orifice of your sexual partners (if they had unprotected sex with you).
full blown AIDS as you've learned about is the manifestation of the symptoms after contracting the HIV years ago, good to note some carriers don't exhibit these symptoms with the cocktail of drugs they consume on a regular basis.