What does the STD - Transmission mean in MW2?

What does the STD - Transmission mean in MW2?I knifed a guy and it suddenly said "STD - Transmission" and I have no idea what that means.
I looked at the challenges but couldnt find anything... what does it do?
What is it for? Do I get anything from it?

Martyn
Its a viral challenge, you can only get it by knifing someone who has it

Heinzman
If you have the STD title or Infected title, if you knife someone then have them knife you back you will transmit the STD or Infection title to them.

Strip Tease
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Give your answer to this question below! Learn about different types of STDs and the effects on the body in both women and men. Sexually Transmitted Diseases Info Blog.

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