One of Usher's herpes accusers is firing back at his claim she assumed the risk of catching an STD when she got in bed with him -- and says there was nothing for her to assume.

Laura Helm is the accuser from Georgia who's suing him for $20 million -- and in new docs, obtained by TMZ, she says it's preposterous Usher is calling her out, saying she should've assumed STDs were a possibility when they hooked up.

Helm says that means she would have to have knowledge of the specific danger -- and there was no way for her to have that info because Usher kept his trap shut about it. Bottom line: consent to sex does not equal consent to risk of catching STDs.

She's essentially put the ball back in his court.