In the comics, Steve Rogers’ 4F condition was described in vague terms as “frailty,” and his visual depictions usually suggested mal-nourishment, which isn’t surprising given the poverty of his early life. 

The MCU expanded upon this, showing us his medical intake report:

It’s a pretty comprehensive list – asthma (which cannot make the scarlet and rheumatic fevers or the sinusitis or chronic colds any easier to bear), a whole cluster of heart problems which were probably due to rheumatic fever having caused rheumatic heart disease, a generally poor immune system, and a family history of tuberculosis. 

And a lot of these conditions – especially asthma, heart disease, scarlet fever, rheumatic fever – are illnesses associated with poverty.