STIs are sexually transmitted infections and are also called as STDs ie sexually transmitted diseases. These are infections that spread by sexual activity.

If you had sexual relationship with a person who may be a stranger or whose status you do not know, it is recommended to get screened for STIs.
The STIs may not show any symptoms for several weeks or months, hence testing is the safest option.

Ulcer/Sore/Blisters on private parts or around anus or mouth.
Irregular growth (warts) in genital area.
Abnormal smelling and discolored discharge from private parts
Rash/Itching on private areas of body
Pain while sexual intercourse
Vaginal bleeding or spotting after sexual intercourse
Lower abdominal pain
Pain or swelling of glands in genital area

Bacterial STIs can be easily cured with antibiotics if treatment begins early enough.
The viral STIs are very effectively managed with modern medicines. HIV is easily managed with ART provided it is detected early. People with HIV whose viral load becomes undetectable with ART can expect normal to near normal life spans.
It is possible to cure Hepatitis C. Hepatitis B can also be effectively managed.
Fungal and protozoal STIs can usually be cured too.

According to WHO , 1 million (10 lakh) STIs occur every day in the world. The good news is, with early detection these can be easily cured or managed.