– cause disease in agricultural crops which lead to reduction of yield and quality of food and cash crops, e.g. tobacco mosaic, cucumber mosaic, groundnut rosette, cowpea mosaic, viruses, etc.
– causes disease and death to man’s domesticated/food animals, leading to losses in poultry, cattle, sheep and other livestock, e.g foot and mouth disease of cattle, red water fever of cattle, Newcastle and  gumboro disease in poultry, rabies in dogs, etc.
– cause disease in man that can lead to death. E.g. rabies, small pox, influenza, yellow fever, measles, AIDS, etc.
– useful in scientific research molecular biology. E.g in the study of genetic recombination processes, mapping of gene, study of DNA and RNA replication, etc.