I consider this a design oversight, but it is very annoying and immersion-breaking when a pawn who has flu or even, hell, plague, loses ability to walk and has to be "carried" instead.
I get it, he would move more slowly, but he can still walk.
I've never seen this. In fact I had a pawn get sick while walking alone and they still made it back.
Does your pawn have "Weak" or other problems? Maybe it's a combination of stuff.
Pawns with diseases can walk, with the same conditions as walking in the town/base map. If it's a minor infection, they'll be slowed down, but still walk on their own. Your pawn probably had something else combining with the flu, to knock him down.