STIs can be caused by one of three things:

Bacteria:
eg, gonorrhoea, chlamydia and syphilis

Infections caused by bacteria are usually easily cured with antibiotics.

Viruses:
eg, HIV, herpes and the liver infection hepatitis

Viruses are harder to treat but with time your body often gets rid of some viruses on its own. Other viruses such as HIV cannot be cured. You can be vaccinated against some viruses, eg, hepatitis A and B.

Parasites:
cause two STIs: pubic lice and scabies

Both can be caught without having sex - eg, from bedding and towels - but this isn’t common.

Symptoms of STIs:
Some STIs can cause symptoms within a few days.

Symptoms of others may not show for days, weeks or months. Sometimes you may notice no symptoms at all or mistake them for something else.

Whether you have symptoms or not, a sexual health check-up will detect any infections.

How can I stay safer?
Using a condom or internal condom cuts the chance of getting or passing on STIs - the condom or Femidom is the only type of contraception that offers any protection against them.

Condoms and Femidoms don’t take away all the risk, however, as they may not cover the part of the body where the STI is (such as a herpes sore or a syphilis rash). Also, some STIs are spread during types of sex where people are not likely to use condoms or Femidoms, eg, oral sex.

You can still get an STI if you have very few sexual partners - but the more sexual partners you have, the more likely you are to have sex with someone with an infection.

These reduce the risk of STIs being passed on:

1. using condoms or Femidoms  
2. having fewer partners  
3. being checked for STIs

Source : Terrence Higgins Trust