Anemia!
Usually, these people cannot run a marathon, lack of useful red
blood cells. They have lower than average oxygen level in the
blood. It is better to downgrade as low as possible. When you
fainted during a run, only God can save you.
Check it out with doctor. It is very important to screen your
partner when married and have children. It will result
in abnormal child.
The advantages, you are naturally protected against
Malaria.