I don’t know, we’re straying into speculative territory here. I like to work from the strips, and base my interpretation on the facts presented. Personally, I find it hard to believe that America wouldn’t know what a cold is, let alone being able to experience the symptoms and yet not link it to illness. 

[Just a side note I forgot to mention before]: 

In “In just 2 minutes you can grasp the exterior of the European economy,” we learn that the nations get sick as a result of recessions. It doesn’t say anything on whether or not they can get sick as a result of exposure to it. 

However, I concur, as @guiltipanda pointed out, Romano contracted Chorea as a result of its prevalence within Southern Italy. But, my addition to that point would be that it has to be a significant political/societal problem as stated in the strip listed above. Remember that the nations have a stronger immune system than your average person.

In other words, the nations can contract illnesses from their people, it just has to be major.

[My speculation] While we haven’t seen it depicted where America directly views illness within his population, he would have had to. 

1) Consider previous leaders who died of illness.
2) Consider how prevalent illness was without modern medicine + during times of war (especially in the trenches).

Likewise, although Canada barred America from seeing England when he acquired a cold after the Revolution, he was still aware of the existence and possibility of personification illness. He’s naïve and inexperienced, sure, but not to that degree. That’s why I would attribute it to both a consistency error and a logistical fallacy. 

As for Hungary, unfortunately, I still have a ton of requests to get to beforehand! I’ve often see people claim that her love for yaoi is fanon. It’s…canon, not that it makes it anymore right. It’s not a mischaracterization, but is more of a miscalculation in how she’s depicted.