Is there any diseases/illnesses that can cause a lack of emotion?

Emotion. Emotional numbing can occur often as a response to extreme trauma. An example would be a combat veteran returning home and feeling nothing at the death of a loved one. People with schizoid personality disorder are indifferent to interpersonal relationships and appear to be emotionless, often. These are just two examples, there are others.
Many. Any disease that affects the brain. Stroke, medications / alcohol, head injury, brain abcess or infection, seizure, tumor, any of the dementias, and of course all the primary psychiatric illnesses.