Hi, and thank you for your question
When we talk about infections being “sexually transmitted” we mean that these infections are typically passed from one person to another during some form of sexual contact. This means that these infections are not generally passed through casual human contact such as shaking hands or hugging. However, each infection does have its mode of transmission, or how it is passed from person to person.
Chlamydia and gonorrhea can be present in semen, vaginal fluid, and rectal fluid so can be passed through oral, anal and vaginal sex. These infections are not passed by contact with blood and are typically not passed through skin to skin touching.
HIV is present in both blood and body fluids such as semen, vaginal fluid, rectal fluid and breastmilk. So it can be passed through anal and vaginal sex as well as through exposure to blood on an open sore or mucous membrane.
Syphilis can be passed quite easily by skin to skin contact with the fluid from a syphilis lesion (blister).
Other STIs such as herpes and human papillomavirus, or HPV (the virus that causes genital warts and some types of genital cancers) are very easily passed from person to person through skin to skin contact.
For more information on how STIs are passed have a look at our STIs at a Glance chart found here: https://smartsexresource.com/sites/default/files/Smartsex_SUPERCHART_2014_v2.pdf
These Know Your Chances charts are also great for understanding which infections are commonly passed through different types of sexual contact: https://smartsexresource.com/about-stis/know-your-chances-0
Let us know if this does not answer your question or if you have any more questions or concerns.