Which of the following are symptoms shared by both the flu and pneumonia?

The shared symptoms between the flu and pneumonia that include diarrhea and fatigue can be attributed to the systemic inflammation and stress that both illnesses place on the body. During a viral infection like the flu, the immune response can lead to feelings of exhaustion and general malaise, which may also manifest as fatigue. In pneumonia, especially when it is caused by a viral infection, fatigue can result from the body's effort to combat the infection, along with the body's increased demand for energy.

Additionally, while diarrhea may not be the most common symptom in pneumonia, it can occur as a result of viral infections or in response to systemic illness. Since both conditions can compromise the body's functions and impact gastrointestinal systems especially in compromised individuals, diarrhea can emerge as a non-specific symptom when experiencing either illness.

The other answer choices involve symptoms that are either not typically associated with both the flu and pneumonia or do not directly highlight the overlap as effectively as diarrhea and fatigue do, as these symptoms provide a clearer representation of the common effects on the body during these illnesses.