There are many myths about how STIs spread. The vast majority require intimate sexual contact - vaginal, anal or oral sex - or, for some, contact with infected blood. They do not survive well on surfaces, so you cannot catch chlamydia, gonorrhoea, HIV or most other STIs from toilet seats, towels, cutlery, swimming pools or door handles.
Kissing is a different matter for a few infections: oral herpes (cold sores, HSV-1) can spread through kissing, and syphilis can occasionally be transmitted if there is direct contact with a sore. HIV is not transmitted through saliva or casual kissing.
Understanding the real routes of transmission helps reduce unnecessary anxiety and focuses prevention where it matters - condoms, vaccination, testing and honest conversations with partners. If you are worried about a specific exposure, a licensed doctor can tell you whether testing is warranted.