Microglia
Research has discovered dystrophic (defective development) human microglia. "These cells are characterized by abnormalities in their cytoplasmic structure, such as deramified, atrophic, fragmented or unusually tortuous processes, frequently bearing spheroidal or bulbous swellings." The incidence of dystrophic microglia increases with aging. Microglial degeneration and death have been reported in research on Prion disease, Schizophrenia and Alzheimer's disease, indicating that microglial deterioration might be involved in neurodegenerative diseases. A complication of this theory is the fact that it is difficult to distinguish between "activated" and "dystrophic" microglia in the human brain.
Human interactions with microbes
Since microbes were not known until the Early Modern period, they appear in earlier literature indirectly, through descriptions of baking and brewing. Only with the invention of the microscope, as used by Robert Hooke in his 1665 book "Micrographia", and by Antonie Van Leeuwenhoek in the 1670s, the germ theory of disease, and progress in microbiology in the 19th century were microbes observed directly, identified as living organisms, and put to use on a scientific basis. The same knowledge also allowed microbes to appear explicitly in literature and the arts.