Final answer:
Norovirus is the most common virus causing sporadic cases and outbreaks of foodborne illness because it leads to stomach and intestinal inflammation.
Step-by-step explanation:
The most common cause of sporadic cases and outbreaks of foodborne illness is Norovirus. This virus is responsible for inflammation of the stomach and intestines and can lead to significant outbreaks, particularly in places like cruise ships. Other viruses such as Hepatitis A, Rotavirus, and Astrovirus also contribute to foodborne illnesses but are less common than Norovirus. Hepatitis A causes inflammation of the liver and requires rest and diet changes for treatment. It is also important to understand that foodborne diseases can be difficult to trace, especially if they have a long incubation period.