Big Brown Bat Nursery Colony - Pictured here is a Big brown bat nursery colony used by females to rear their young. The size of Big brown bat colonies range from 20 to 300. Big brown bats are significant predators of agricultural pests. The little brown bat is a colonial species, with hibernating colonies consisting of up to 183,500 individuals, though the average colony size is little more than 9,000. Bats have been shown to carry a number of diseases and harmful infections, including rabies and viruses related to SARS (severe acute respiratory syndrome). Moreover, research suggests bats may be the original hosts of nasty viruses such as Ebola and Nipah, which causes deadly brain fevers in people. Big brown bats will bite humans if they feel threatened, are provoked or sick. Even though sick bats are more likely to be submitted for testing, in 2011, only 3.8% of submitted big brown bats were positive for the rabies virus. While these bats are not aggressive creatures, they are better left alone.