Std or Sexually Transmitted Diseases are very easy diseases to get, the most dangerous and famous is HIV, but there are a lot of others that can be pretty bad, there are countries where people have more diseases than others and you need to be aware of what you are doing all the time, most of the time not even using the condom can save you, I have some tips to stay FREE:
Don't eat or drink from anyone else bottle or plate, not even you mum bottle. It's just more hygienic and simple to do.
Don't touch any blood
Protect always your eyes in ever "danger situation", blood contact through eyes is the most effective for transmission
Be smart choosing your sexual partner, dig into her past a bit and figure out if she's a smart person or just a stupid.
Study the country your going and the people behavior there.
Look up what are the most popular diseases of the town or country your visiting and take precautions based on this particular disease.
Go test your self every 6 months, HIV test and also std's test are free almost everywhere even in the US. You can find your local store allowed to test HERE.
Before putting your penis or taking a penis, look up the conditions and be able to figure out if this person might have some std's, like vaginas that smells always bad and have white secretions are bad, penises with weird tip color and white secretions are usually bad.