Hasn't it already been established that the sickness is from the pigs?
Also a flu can be pretty tough, yes not coughing up blood tough. I think the word flu has become a common word for almost any kind of sickness recently. People now say they had a "24 hour flu bug" and things like that. The actual flu, influenza is a pretty rough sickness that puts you under for a few days with the sever sweats, pain, fatigue, etc.
Yes one doesn't typically cough up blood with the flu as much as they do on the show, but knowing that everyone is infected with the walkers disease even before they die I don't think it is too much of a Hollywood stretch to say any coughing or internal unrest can be significantly worse than it more typically would be.
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