Sunlight and no people forced locked inside their homes. One of the reasons it disproportionately affects the poor is because of vitamin deficiency (D). Combine the two; nutrient deficiency and lack of sunlight exposure and you get people susceptible to this and many other diseases. The reason it doesn't touch the entire continent of Africa is because the majority of the population has to go outside daily for food and work. Lock them up for a few weeks and even they'd get outbreaks.