How does STD transmit?

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how does STD transmit? and HIV ? Having sex without condom ? Or more than 1 sex partner?

Quite simply, it transmits from an infected partner during unprotected sex.

so, a virgin boy and a virgin girl have sex , then there won't be STD right ?

Not 100% guarantee but remember, you have to have an STD to transmit it.My earlier response used the term unprotected sex. If one is clear of an STD, he or she is unable to transmit in unprotected sex, but a pregnancy could still result. Be smart!

You can't catch an STD if the only contacts you have are with people who don't have STDs.

However, it's important to recognise that STDs are called Sexually Transmitted Diseases because they are mainly transmitted by sex, not exclusively by sex. They usually can be transmitted in non-sexual ways as well, so a virgin could conceivably have an STD already. For example, AIDS (and most other STDs) can also be transmitted by direct blood-blood contact, e.g. from shared hypodermic needles. Herpes and genital warts are transmitted by simple skin contact, which can occur without actual sex.

People get STD mostly by having sex with someone who has the disease. "Having sex" means anal, vaginal, or oral sex. HSV-1 and HSV-2 can be found in and released from the sores that the viruses cause. The viruses can also be released from skin that does not appear to have a sore. Generally, a person can only get HSV-2 infection during sexual contact with someone who has a genital HSV-2 infection. Transmission can occur from an infected partner who does not have a visible sore and may not know that he or she is infected.
HSV-1 can cause sores in the genital area and infections of the mouth and lips, so-called "fever blisters." HSV-1 infection of the genitals is caused by mouth to genital or genital to genital contact with a person who has HSV-1 infection.