[Dallas59]

There are many possible causes of unexplained weight loss, including undiagnosed celiac disease and thyroid disorders. Potential causes of unexplained weight loss include:

Addison's disease
Cancer
Celiac disease
COPD
Crohn's disease
Dementia
Depression (major depression)
Diabetes
Heart failure
HIV/AIDS
Hypercalcemia
Hyperthyroidism (overactive thyroid)
Hypothyroidism (underactive thyroid)
Parkinson's disease
Peptic ulcer
Tuberculosis
Ulcerative colitis

[ImJustAnotherOkie]

You left off exercise.

[rabidralph]

Don't forget flesh-eating bacteria.