List of diseases that include lethargy as a symptom. Any major, well-known medical conditions associated with lethargy are included below, listed alphabetically with photos when available. Any maladies, sicknesses or illnesses with lethargy as a symptom can be seen below. For any real, further diagnosis of something you feel you might have, please consult a doctor. This list includes information about each disease, such as what risks are associated with it. Illnesses here include everything from Meningitis to Dehydration.This list answers the questions, "What risks are associated with lethargy?" and "What disease can I have if I have lethargy?"