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Age: highest prevalence in children 6M to 5Y Diet: carotenoids are highly available but require fat for absorption Disease: infectious disease contribute to depletion. Diarrhea decreases absorption, fever increases urinary losses and metabolic utilization rates Measles specifically affect vitamin A balance - absorption and utilization Seasonality: seasonality affects both the availability of foods and the incidence of infectious disease Cultural: in certain cultures use of green leafy vegetables or mangoes is taboo. There are also cultural influences regarding feeding of infants and pregnant women Clustering: clusters exist on national, neighborhood or household levels