Some are not. If you have sex with someone who has an STD, you can get it, too. Many people who have an STD don't know it. Some people won't tell you, even if they know. One in four STDs occurs in teenagers.
Herpes sometimes can cause painful sores that keep coming back.
Some HPV in women can lead to cancer of the cervix. HIV and hepatitis B can kill you. Teenagers' immune systems are still developing and not able to fight off diseases like adults' immune systems do. Teenagers are experimenting with a lot of things during these years, like drugs, alcohol and sex. Sometimes, under the influence of drugs and alcohol teenagers take risky chances, like unprotected sex.