• They are extremely small and can only be seen in an electro-microscope
• When out of cell they are inactive , they can only became active (alive) when they are in a living cell
• Their genetic material is either RNA or DNA and never both
• Those with DNA are called DNA viruses and those with RNA are called RNA viruses
• They are highly specific as to which type of cell they can invest
• The cell organelles are absents
• They have no nucleus, therefore they are called eukaryotic, they have no cytoplasm and no cell wall
• They are obligate intra cellular parasite
• They cannot reproduce on their own therefore they most invest living cells
• Viruses are considered as non living but they have one living characteristic which is the ability to reproduce. Their non living property include the ability to crystallize (like chemicals)

Viruses therefore show both living and non living characteristic that is why they have not yet be able to classify viruses into any of the five kingdoms.