(a) Cryptospordisiosis rates in Worcester, MA, for demographic sub-populations in 1995. Other than the slight delay in development of symptoms for adults, each sub-population in Worcester was found to be equally affected. (b) Giardiasis rates in Worcester, MA, for demographic sub-populations in 1995. Arrows highlight areas suggesting that the effects of the disease on the different sub-populations of the disease were not always in constant proportions. These differences would go unnoticed using average mass action transfer terms.