A large proportion of STIs are 'silent' - they cause no noticeable symptoms, especially in the early stages. Chlamydia, for example, is asymptomatic in most women and many men; gonorrhoea, trichomoniasis, HPV and even HIV can go unnoticed for a long time.
The absence of symptoms does not mean the absence of infection. Untreated STIs can still be passed to partners and can cause complications such as pelvic inflammatory disease, infertility or chronic infection. This is precisely why routine screening is recommended for sexually active people, particularly after a new partner or unprotected sex.
If you are sexually active, talk to a licensed doctor or sexual-health clinic about how often you should be screened based on your age, number of partners and risk factors. Testing is confidential and, for many infections, simple - a urine sample, swab or blood test.