An abscess is best described as:

An abscess is best described as a localized collection of pus that typically forms as a result of infection, inflammation, or blockage of a duct or gland. This collection of pus, which is the result of the body's response to infection, leads to the formation of a swelling that can be painful and tender to the touch.

In this context, describing an abscess as a papule containing pus captures the essence of what an abscess is. It highlights the presence of pus, which is a key characteristic of an abscess, and distinguishes it from other conditions. Papules are small, raised bumps on the skin, and when one of these develops a pus-filled cavity due to infection, it fits the description of an abscess perfectly.

The other choices do not accurately define an abscess. A swelling due to an insect bite refers to a different type of inflammatory response, while cellulitis is a more extensive bacterial infection of the skin and underlying tissues, which can lead to abscess formation but is not itself an abscess. Lastly, a viral infection does not result in the formation of pus, making it unrelated to the definition of an abscess.