Diagnosis of viral infections includes direct identification of virus, virus isolation, and measurement of serum antibodies. However, most viruses require cell culture for replication. A progeny virion assembled during the infectious cycle is the vehicle for transmission of the viral genome to the next host cell or organism, where its disassembly leads to the beginning of the next infectious cycle. They have no capsid or envelope and they are associated with certain plant diseases. Virusoids are dependent on the presence of virus replication for multiplication, and are packaged into virus capsids as passengers.
Prions are infectious agents that consist of a single type of protein molecule.
They have no nucleic acid component. The practice of variolation spread through Asia Minor, England, and into the USA when in 1776, George Washington introduced the practice to the Continental Army soldiers. 1796 Edward Jenner s smallpox vaccination experiment. He demonstrated that inoculation of a young boy with extracts from cowpox lesions induced mild symptoms and protected from smallpox. Koch s postulates.