Economy: 

Whether it’s due to war or a stock crash, the nations will
develop colds during economic recessions.

Their physical strength is dependent on their economy.

Age: 

While the nations are immortal, they do age, albeit gradually.

Ex: China is commonly referred to as an old man/ grandpa and
tires easily.

Ex: Japan can’t do strenuous exercise, also makes a constant
mention of his old age, and suffers from health complications as a result of
his high sodium intake.

Weight: 

The nations’ weight is influenced by the average weight of their population.

If obesity levels rise, they’ve been seen to gain weight.

However, America seems to be the exception to this rule due
to working out excessively and trying all sorts of diets and exercise machines.
In reality, the extra weight he perceives is actually just muscle.

Anatomy:

A nation’s blood type is dependent on the most common one
within their population.