STD? Depende sa STD.
HIV is a STD btw. Usually, walang symptoms. If meron, around 50% will have flu-like symptoms for like a week - usually a few weeks to a month after exposure. Then gagaling. Then you don't have one until it gets worse, for like a few years - sometimes until mag evolve siya to AIDS. But after you get it, you can now pass it to someone. Mas nakakahawa during the first few months kasi mabilis mag multiply yung virus.
Getting tested is the only reliable way to tell if you have HIV.