Page 91 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

General Properties of Viruses

• They are the smallest infectious agents known

• They infect man, animals, plants and bacteria.

• In contrast to bacteria, fungi and parasites, viruses are not cells.

• They contain one kind of nucleic acid (RNA or DNA).

• They are obligate intracellular parasites → hence they cannot grow in artificial

medium

• They are metabolically inert.

• Viruses vary considerably in size and shape. The smallest viruses are about 0.02

micrometers (20 nm) as poliovirus while the largest viruses measure about 0.3

micrometers (300 nm) as smallpox virus

• Virions: The extracellular phase (Complete intact virus)
possess few if any enzymes and cannot reproduce independent of
living cells.
Prions: (proteinaceous infectious particle) An infectious agent that
causes

neurodegenerative disease.

What is Viroid???

General Microbiology & Immunology (PM 401) theoretical handbook Page 80