Above: The Virus. Credit: Dr Fisher CDC.
An infectious viral pathogen typically enters the body by inhalation of infected droplets, or from-hand-to-mouth (or eyes) after touching a contaminated fomite (a door-handle or a cutlery utensil, etc.), or via sexual contact. From there the virus must reproduce within the body, and then pass these copies of itself on to others, e.g., by causing the human to cough and sneeze.
Parallels between the viral photo and the pathogen continue through each element of this three-aspect life-cycle: the virus aims to gain a host (entry), then it must copy itself (reproduction), and then spread to further hosts (known medically as shedding). Equally, the Internet meme or viral photo must also cause the user to become aware of it visually (entry), to copy it (reproduction), and then pass it on to other Internet or social media users (shedding).
The function of the content of a viral photo, given the above parallels, should not then be adjudged aesthetically, but rather by a single basic critieria: survivial. Is the photo so amusing, or shocking, that a recipient will be motivated enough (having seen it) to send it on electronically to at least one further computer user?
(7 February 2018, revised 4 January 2019)