Sexual disease video
Sharpton said if she had known that Usher had herpes she wouldn't have had sex with him.
"I feel like my rights were violated," she said, adding she decided to join the lawsuit so he "does not do this to anyone else".
"You need to warn your sex partners..they can make their own decisions," she said.
But once the documents were made public, a woman who said she slept with Usher in April filed a million (£15.4million) lawsuit against him for giving her the disease.
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Free 5-day trial As if you haven't had enough of what I'm about to tell you in various high school and college classes: practicing safe sex is very important.
It adds the alleged victims have suffered severe emotional distress, birthing complications, stress, depression and anxiety about contracting or being exposed to herpes.
Court filings claim that in the event that Usher doesn't have herpes, his conduct has still caused "severe emotional distress" since he has allowed them to worry that he has exposed them to the virus.