A viral disease is an illness caused by a virus, which is a tiny infectious agent that can only replicate inside the living cells of an organism. Viruses can infect various hosts, including humans, animals, and plants, leading to a wide range of symptoms and health issues. Common examples of viral diseases include influenza, COVID-19, and measles. Transmission of viral diseases often occurs through direct contact, respiratory droplets, or contaminated surfaces. Vaccines are available for some viral diseases, helping to prevent outbreaks and protect public health. Good hygiene practices, such as handwashing and avoiding close contact with infected individuals, can also reduce the risk of infection.