In addition to bacteria, several other groups of microorganisms, including viruses, fungi, and microsporidia, also cause diseases in insects. Microorganisms utilize invertebrates for food just as they attack plants and other types of animals. Their relationships with hosts vary from obligate pathogens, which do not grow outside of the hosts in nature, to facultative pathogens, which only live as pathogens when an opportunity presents itself. The major microbial groups attacking invertebrates are roughly the same as those that have adopted life styles as pathogens of vertebrates and plants. As you know, virtually all species of pathogenic microorganisms infecting humans do not infect plants. Similarly, species of microbes causing disease in invertebrates generally specialize on invertebrates. The pathogens vectored by insects, such as malaria and plant pathogenic viruses, are special cases. In fact, within the invertebrates, pathogens display host specificity for certain groups and this is especially true of obligate pathogens that have close associations with hosts.

In some ways, pathogens of invertebrates have easier hosts to overcome than pathogens of vertebrates. The hard exterior cuticle of insects and mites poses a formidable barrier to microorganisms. If a pathogen enters the body of an insect or mite, these invertebrates then have an immune response for protection. However, invertebrate immune systems are quite different from vertebrate immune systems and are not as powerful. Numerous pathogens have developed the ability to overcome their invertebrate hosts and utilize the entire invertebrate body as a source of nutrients for microbial reproduction.