Is there a disease that causes muscle weakness and weight loss?

Yes: Generalized muscle weakness is a sign of a significant illness (auto-immune or thyroid diseases are possibilities). Unintentional weight loss is another sign of a significant problem (cancer, thyroid problems, psychological issues, or auto-immune diseases are possibilities). A person with both symptoms should be at the doctor's office getting a full evaluation.

Yes: Systemic illness, toxicity, congenital illness to name a few can cause muscle weakness and weight loss.