A virus is a microscopic infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

Key characteristics of viruses include:

• Size: Viruses are much smaller than bacteria, typically ranging from 20 to 300 nanometers in diameter.
• Structure: A typical virus consists of a nucleic acid genome (DNA or RNA) surrounded by a protective protein coat called a capsid. Some viruses also have an outer lipid envelope.
• Replication: Viruses are obligate intracellular parasites, meaning they cannot replicate on their own. They must invade a host cell and use the host cell's machinery to produce more virus particles.
• Infection: Viruses cause a wide range of diseases in plants and animals, including humans. These diseases range from mild illnesses like the common cold to severe and life-threatening diseases like Ebola and AIDS.
• Evolution: Viruses can evolve rapidly due to their high mutation rates and short generation times. This allows them to adapt to new hosts and evade the immune system.

For more detailed information, you can refer to these resources:

• National Center for Biotechnology Information (NCBI) - Chapter 2 Introduction to Viruses
• Science Learning Hub - Viruses