Infectivity: Viruses are able to infect living organisms and replicate inside them. They invade cells and use the cell's machinery for their own replication and spread.
Genome: Viruses possess a genome composed of either DNA or RNA, but not both. The genetic information of a virus is essential for its replication and infectivity.
Non-cellular structure: Unlike cells, viruses do not have a typical cell structure with a defined nucleus, cytoplasm, or organelles. They are acellular entities consisting mainly of a protein coat (capsid) and the genetic material.
Parasitism: Viruses cannot reproduce on their own. They are obligate intracellular parasites and require a suitable host cell to replicate and survive. They invade living cells, hijack the cell's functions to make copies of themselves, and then release new viral particles into the environment.