They have to want the one you have to get bitten. If they want any other virus then it will be greyed out.

i'm 95% sure i know what's causing this, if so it will be fixed in just a few minutes! :)

[edit] fixed, but not retroactively. anyone who has already signed up for a 2016 virus still can't be bitten. but anyone who signs up from here on out can be.

they can't be bitten and will get processed through the queue.

people who signed up for the free random option can be bitten by any mature zombie.