are infections that can be passed from one person to another via any method of sex, including the genital tract, the mouth, and the saliva. Many people who have them don't even realise it because they're so common and cause no symptoms at all. Serious health issues can develop from sexually transmitted diseases if they are not treated. However, the good news is that testing is simple, and most STDs are straightforward to treat. Causes : An STI is the root of all sexually transmitted diseases. Vaginal, oral, and anal intercourse are common transmission routes, but they can also be spread by direct skin-to-skin contact. If caught early enough and treated, some of these conditions never manifest into full-blown illnesses. However, if the bacteria that caused the infection also destroy cells and affect the body's processes, the STI will develop into an STD.