Like all diseases STDs are transferred from organism to organism. Most often by sex but many are communicable through more casual contact or through outside vectors.

Okay, so S.T.D.s are transferred from organism to organism most often by sex; also when you said "many are communicable through more casual contact or through outside vectors" does that also mean the same thing as sex or something else? Please explain/help me!

I mean casual in the relationship-sense, non-sexual contact like kissing, toilet seats, sharing drinks, blood-to-blood contact. A virus doesn't have a brain, it doesn't know if you're having sex or not. It just looks for an opportunity to spread to unexposed tissues or blood. STDs are evolved to spread through sexual contact but the reasons they haven't died out as viruses is that they are the most communicable virulent strains. They will find a way to spread even without sex.

It's actually very rare for an STD to be transmitted from a toilet seat and only a very few of them can be contracted that way (crabs, or pubic lice, for example). Most likely, if you have an STD, you got it from sexual contact and the person you got it from did, too. But, like I said, it does depend somewhat on the type of STD.

You should also know that oral sex transmits all the same diseases as vaginal sex. For example, if you give oral sex to someone with herpes, you can get it on your mouth and also give it to the next person you kiss or give oral sex to. The take-home message is to use protection (condoms, dental dams, etc) when you have sex, especially if you don't know for 100% certainty that both you and the other person are free of STDs. Sex is an important part of life, but it's not worth dying for.

As for your other question, yes, doctors can test you for STDs. If you wish to be tested, you need to ask about it.

Can all S.T.D.s kill you; or is it just some that can; or what?

No, not all of them can kill you. Some of them can be really serious, or even deadly, like HIV, Hepatitis, Syphilis, HPV (Genital Warts--can cause cervical cancer in women; throat and neck cancer in men). Luckily for a lot of people, if they are caught early, they can be managed with medication or the cancer can be found early (or in the case of Syphilis, cured), so they don't have to be deadly. However, too many people don't get checked and end up spreading it to other people.

Other STDs can cause other problems. Chlamydia is treatable, but many people don't know they have it. If a woman goes too long without treatment, it can cause infertility. Herpes won't kill you, but even with drugs to suppress it, you can still spread it.

Unfortunately, there is no way to predict if you are going to get a curable or an incurable STD. That's why you need to protect yourself from all of them by using condoms and dental dams and getting regular screenings for you and your partner(s) if there is any chance of exposure.