Final answer:
A manager should exclude a food handler from an operation if the handler exhibits symptoms of diarrhea, as it can indicate an infectious condition caused by various pathogens that are transmissible through food.
Step-by-step explanation:
When caused by an infectious condition, the symptom that should cause a manager to exclude a food handler from an operation is diarrhea. Infectious organisms, such as bacteria like Vibrio cholerae, Listeria monocytogenes, Staphylococcus aureus, and viruses like norovirus and rotavirus, as well as parasites like Giardia lamblia, can induce this symptom. These pathogens can be transmitted via contaminated food and can cause severe gastrointestinal illnesses with symptoms such as nausea, vomiting, cramping, and diarrhea. Since diarrhea is highly infectious and can easily contaminate food and spread to others, it is imperative to exclude any food handler showing this symptom from a food operation to prevent a potential outbreak.