I just started Season 4 of the Walking Dead, so I know that Wood berry is flattened and all the civilians there moved to the prison to be with Rick, Darrel, Maggi, Glen etc, one of the refugees is a guy with glasses named Patrick.
I just watched S4E04 (I think it was) and found Patrick constantly coughing, he excused himself from the knives class and staggered into the toilets where he slouches over the sink and coughs.
After a while, a woman heads into the same toilet and hears a noise behind her, she goes to investigate and finds nothing so she leaves in a hurry...Patrick slowly follows... Only he isn't human, he's a walker.
Then all hell breaks loose in the cell block where quite a lot of people died.
How did Patrick end up with the virus in the first place? Especially as it did spread.