This extreme response to fava beans is called favism, and is associated with low levels of an enzyme called glucose-6-phosphate dehydrogenase, or G6PD.
Fava beans, also known as broad beans, can cause some people to experience a sudden, rapid breakdown of their red blood cells — a condition known as hemolytic anemia.
So, while people with the G6PD deficiency might have to give up fava beans at the dinner table, they might be a little safer when the mosquitoes come out to bite.