VIRUSES: Overview Viruses are obligate intracellular parasites, which massively multiply by diverting or"hijacking the host cell biosynthetic machinery Viruses were first defined as filterable infectious agents based on their small size and ability to cause disease (In 1892, Dr Iwanowski reported that filtered extracts of tobacco leaves with mosaic disease could still infect other plants) Viruses can range from the very small 20 nm to the near bacterial-200 200X 250 nm for the pox viruses such as vaccinia, first described by light microscopy by Dr. Buist in 1887 All classes of living organisms studied to date are the host to VIrusesVIRUSES: Overview • Viruses are obligate intracellular parasites, which massively multiply by diverting or “hijacking” the host cell biosynthetic machinery. • Viruses were first defined as filterable infectious agents, based on their small size and ability to cause disease (In 1892, Dr. Iwanowski reported that filtered extracts of tobacco leaves with mosaic disease could still infect other plants). • Viruses can range from the very small ~ 20 nm to the near bacterial – 200×200×250 nm for the pox viruses such as vaccinia, first described by light microscopy by Dr. Buist in 1887. • All classes of living organisms studied to date are the host to viruses