Question

STIs are a major cause of ______.

Answer 1

Https://www.nichd.nih.gov/health/topics/stds/conditioninfo/Pages/causes.aspx 
HIVs since there is bacteria including chlamydia, gonorrhea, and syphilis which causes viruses such as HIV or AIDS 

Answer 2

Well, they are a major cause of HPV. There are differents groups of STIs. On one side we have the bacterial STIs, and on the other we have the viral STIs. Hope this helps.