The Mario Virus was the main antagonist of the episode, SMG4: Mario's Inside Story. As it's name suggests, it is a infectious virus those viral pathogen is in the shape of Mario.
It originally resides in the old spaghetti Mario found behind the bathroom toilet in the basement, which according to Luigi, was probably there for at least 10 years. Mario unsurprisingly ignored Luigi and ate it, which resulted him to be food poisoned.
The virus nearly killed Mario, but it was ultimately killed by Mario's Brain, resulting in Mario now being cured.
The virus may be inspired by SARS-CoV-2 virus, due to the episode being made during the 2019-20 coronavirus pandemic.
Though SARS-CoV-2 causes a respiratory virus, which causes respiratory illness rather a food-borne illness. The virus is mainly spread during close contact and by small droplets produced when infected people cough or sneeze. And currently, there is no known possible evidence of transmission of SARS-CoV-2 by food.
Technicality, the Mario Virus might be debatable considered to be a Norovirus because most cases of food-borne illnesses are often caused by viral infections and Noroviruses are a common cause of viral food-borne illness.
It is probably a parody of Thrax from Osmosis Jones.
