Caterpillars are often killed by diseases caused by naturally occurring bacteria, fungi, or viruses. Caterpillars killed by viruses and bacteria may turn dark, and their bodies may become soft and limp. These carcasses, hanging limply from foliage or twigs, eventually degenerate into a sack of liquefied matter. When broken, they release more viral particles or bacterial spores that infect other caterpillars that eat contaminated foliage. Such disease outbreaks can rapidly reduce populations under favorable conditions, although outbreaks are difficult to predict and may not occur until caterpillar populations have become high.
