Virus

A virus is a tiny infectious particle consisting of genetic material (DNA or RNA) enclosed in a protein coat (capsid). Viruses are not living cells - they have no cellular structure or metabolism and can only replicate by infecting a host cell and using the host-s machinery to produce new viruses.

• Not alive: Viruses are acellular and lack organelles; they cannot grow or reproduce on their own (they must hijack a host cell to replicate).
• Genome: A virus contains either DNA or RNA as its genetic material (unlike living cells, which have DNA as genome and use RNA for protein synthesis).
• Size: Viruses are much smaller than cells - typically ~20-250 nm - too small to see with a light microscope (about 100- smaller than bacteria).

• Common comparison - Viruses vs. bacteria: Expect questions noting that viruses are non-living (cannot reproduce without host, not killed by antibiotics) whereas bacteria are living cells that can reproduce independently.
• Genome question - Many exams highlight that viruses can have an RNA genome (e.g., retrovirus) or DNA genome but not both, distinguishing them from cellular organisms.
• Structure clue - If a question describes an infectious agent as "genetic material in a protein coat without its own metabolism", that-s pointing to a virus (not a bacterium or other microbe).