Which of the following is a common symptom of viral infections?

Fever is a common symptom of viral infections because it is part of the body's natural immune response to fight off viruses. When the body detects an infection, the immune system releases substances called pyrogens that cause the body's temperature to rise, creating a less favorable environment for the virus to thrive. This elevated temperature can also enhance the activity of immune cells that combat the infection.

While hair loss and chronic coughing can be associated with certain conditions or specific viral infections, they are not general symptoms of viral infections, making fever the most recognized and prevalent indicator.