According to the CDC, one in five Americans have a sexually transmitted disease (STD). However, rates of infection vary by age, sex, and state. For example, young adults between the ages of 20 and 34 have the highest rates of infection.
There are 4 Viral STI, all of which are very common and incurable.
Human papillomavirus (HPV) is a group of viruses and the most common STI, it causes genital warts and some type cause cancer in both men and women.
Then there is HSV which causes Herpes which affects one out of every six people in the U.S. age 14 to 49.
There is also Hepatitis B that affects the liver and can cause acute or chronic disease.
And finally HIV which causes AIDS.
It doesn't matter where you are sexually active, Thailand, the US, the UK, Australia.... and it doesn't matter if the person you're sleeping with looks like a slapper, or they are a nice bit of upper class totty, anyone can be carrying one or more of these viruses and if you are a normal adult whos had in excess or 100 sexual partners then chances are you've slept with someone carrying one of these or you've already contracted one of them.
Which is why you should always use a condom, despite condoms making sex almost pointless for us guys