Viruses are tiny particles that infect organisms such as animals, plants and humans often causing disease. They differ from bacteria because they cannot replicate on their own and require a ‘host’ such as a human to be able to multiply and spread – think of how a computer virus works – it gets into the system, it replicates, it causes damage and then spreads
Viruses cause symptoms of disease when they multiply inside the cells of the host, causing cell damage.