Viral diseases are illnesses caused by viruses, which are tiny infectious agents that can only replicate inside living cells. These diseases can affect various parts of the body and may lead to symptoms like fever, fatigue, and respiratory issues. Common examples include the flu, common cold, and COVID-19. 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, can also reduce the risk of infection.