Which illness is classified as being caused by a virus?

The common cold is classified as an illness caused by a virus. It is primarily caused by rhinoviruses and other types of viruses which lead to inflammation in the upper respiratory tract, resulting in symptoms like a runny nose, sore throat, cough, and sneezing. This viral nature distinguishes it from other types of illnesses, particularly bacterial infections, which are caused by bacteria rather than viruses.

When considering other options, the flu is also a viral infection, so while it is not incorrect to identify the flu as a viral illness, it’s important to note that the common cold specifically pertains to a broader range of mild respiratory symptoms primarily caused by viruses. Bacterial infections, on the other hand, are distinct pathogens that require antibiotics for treatment, and allergies are immune responses that are not caused by pathogens at all. Thus, the common cold represents an important category of viral infections, making it the right answer in the context of the question.