A variety of microbial pathogens including bacteria, protozoans, microsporidia, viruses and fungi are specifically pathogenic to insects and completely harmless to other forms of life. This selective pathogenicity renders many of them valuable as biological control agents of insect pests. Some, such as Baccilus thuringiensis, have been the source of natural insect toxins that are now synthesized as biopesticides, or engineered directly into crop plants. Many insect diseases can be very important sources of mortality in pest populations and lead to precipitous population declines when they become epizootic (analogous to an epidemic in a human population). However, many attempts to induce epizootics in pest population by means of distributing disease spores or other inoculum fail because stringent environmental conditions are often necessary for successful infection and/or transmission of the disease. For example, many fungal diseases of insects require high humidity or prolonged leaf wetness in combination with particular temperatures to infect their hosts. Some success has been obtained with baculoviruses commercially formulated with sunscreens to protect them from solar radiation when sprayed onto plant surfaces. However, most insect epizootics proceed without human assistance when suitable environmental conditions arise. Conservation is also a consideration when epizootics are a significant natural mortality factor in pest populations. For example, excessive use of fungicides to control powdery mildew and other foliar diseases in potatoes can also eliminate insect-pathogenic fungi, thus favoring aphid outbreaks.