Overall, contact lenses are quite safe. People have been wearing and using them for several decades now, with little to no detriment to their vision. However, that does not mean that you cannot get any sort of health problem from contacts. There are, in fact, some very unusual health problems you can get from contacts, but smart preventive measures will help you avoid them.
Herpes of the Eye
Just about any part of the body can contract a type of herpes virus, so it should come as no surprise that your eyeballs can get herpes too. The primary means of developing herpes of the eye is through your cold sores. If you have an open cold sore and touch it, or you rewet your contact lens by putting them in your mouth while you have a cold sore outbreak, you can give yourself herpes of the eye. Likewise, if you borrow a friend's contact lenses and that friend suffers from cold sores, you are running the same risk. Only wet your lenses with saline solution and always wash your hands thoroughly after touching your lips and mouth.
Scratched and Scarred Corneas
Scratching and scarring your corneas are directly tied to wearing your lenses too long and not wetting them and/or sleeping in them. If your lenses are not extended-wear lenses, they can dry up and shrivel into hard bits of plastic. Your own tears keep the lenses slightly moist so they do not dry up all the way, but in this less-than-soft state they can scratch and scar your cornea and even your iris. Do yourself a big favor and rewet the lenses regularly, remove them before you go to sleep, and do not wear them several days in a row without removing them.
Sexually Transmitted Diseases and Eyes
Sexually transmitted diseases can cause their own infections in your eyes. This usually happens when you have contact with an infected person's genitalia and then touch your eyes afterwards. Sometimes the infection can be transmitted systemically, via the blood stream after unprotected sex. Strains of gonorrhea and chlamydia can affect your sight and have both short-term and long-term effects.
When you throw contact lenses into the mix, you create an alternate vehicle for STD bacteria, which can transfer to someone else or cause you to reinfect yourself after a round of antibiotics. Your best bet when you do not know a potential sexual partner's health history is abstinence, followed by condoms, followed by handwashing and then throwing out the contacts you wore during a one-night stand, in that order. For more information, contact a professional like those at The Eye Center.