There can be several reasons why a person's weight may be very low, such as:
1. Genetics: Some people may have a genetic predisposition to be naturally thin.
2. Hyperthyroidism: This is a condition where the thyroid gland produces too much thyroid hormone, which can cause weight loss, among other symptoms.
3. Malnutrition: Poor nutrition, such as not getting enough calories or nutrients, can lead to weight loss.
4. Digestive disorders: Certain digestive disorders such as celiac disease, inflammatory bowel disease, or chronic diarrhea can lead to weight loss.
5. Cancer: Some types of cancer can cause weight loss due to the body's increased metabolic rate.
6. Chronic illness: Chronic conditions such as HIV, tuberculosis, or chronic obstructive pulmonary disease (COPD) can cause weight loss.
7. Medications: Some medications, such as those used to treat cancer or depression, can cause weight loss as a side effect.