Definition: Virions are particles that are small enough to be present in a cell or organism, but large enough to remain undetectable by the immune system. They can be either infectious or non-infectious and have been used as a model for studying viral infections. Infection is when a virus enters a cell and starts replicating within it. Virions are particles that contain the viral genome, which is essential for replication. The virion's surface proteins, which recognize and bind to cellular receptors,