Although a routine bout with the flu or virus results in some weight loss, certain more serious ailments make you lose weight. You might not even be aware of the dropped pounds at first, but if you lose more than 5 percent of your body weight over the course of six months to a year for no obvious reason, consult your doctor, MedlinePlus says.
Kidney disease can cause weight loss because it often involves nausea or vomiting, making it difficult to keep food down. The resulting loss of protein through urine can also make it hard to maintain weight, the Mayo Clinic says.
Any illness that affects your stomach, intestines and digestive tract can make you lose weight by interfering with your body's absorption of necessary nutrients. Chronic diarrhea or an inflammatory bowel disease, such as Crohn's or ulcerative colitis, inevitably results in lost pounds, even if you're eating as much as always, the Mayo Clinic says. Celiac disease causes the immune system to react to gluten, a protein contained in bread and pasta, and causes malfunction of the small intestines if it's not identified and you make diet adjustments. Elderly people are especially prone to weight loss from these ailments, according to American Family Physician.
The body's hormones include more than just testosterone, estrogen and progesterone. The thyroid and adrenal gland both produce hormones that can affect your weight when something causes them to secrete more than is needed, or not enough, MedlinePlus says.
Infections can be caused by parasites or viruses. You may carry them with you unaware for some time before you notice you're losing weight. Infections that potentially cause weight loss include human immunodeficiency virus or HIV and tuberculosis, the Mayo Clinic says.
Not all illnesses are physical. According to American Family Physician, depression is a major cause of weight loss among elderly people, and when Alzheimer's disease is accompanied by weight loss of 5 percent or more, it's generally a precursor to death. Personality disorders such as anorexia and bulimia rob pounds, especially in young people. Parkinson's disease also causes weight loss.