Combined fatigue, weakness, diarrhea, and weight loss can be caused by a variety of conditions, in particular infectious diseases, inflammatory syndromes, chronic diarrheal conditions, or inherited diseases, according to the Merck Manual. The most common causes are infectious, which can be bacterial, viral, or parasitic. The second most common group are the inflammatory conditions that could have a genetical component, but are no necessarily considered inherited diseases. The most rare are the purely genetical conditions, some of which can happen in very young children and are related to mutations in the body's immune system as listed by the Immune Deficiency Foundation.
Infections that cause these quartet of symptoms typically fall in the bacterial or parasitic range, but according to the World Health Organization, viruses such as the West Nile and viral hepatitis are well documented as causes of tiredness, weakness, diarrhea and weight loss. The influenza could be lumped in with these other viral syndromes, but the diarrhea symptom occurs more often in children and less likely in adults, according to the Centers for Disease Control. Bacterial infections that result in these symptoms range from Campylobacter and Salmonella infections to Mycoplasma which typically causes pneumonia. In third world countries or in areas that have been struck by natural disasters, typhoid and cholera commonly produce these symptoms. Parasitic conditions include malaria cryptosporidium, and a variety of intestinal worms including tapeworm and roundworm as listed by the University of Maryland Medical Center.
Two of the main inflammatory causes of fatigue, diarrhea, weakness, and unintentional weight loss are generalized by the Crohn's & Colitis Foundation of America, as inflammatory bowel syndrome, IBS and inflammatory bowel disease, IBD. While IBS produces symptoms similar to IBD, it is a functional syndrome that doesn't necessarily cause inflammation of the mucosal lining in the gut, but it is often associated with conditions such as fibromyalgia and chronic pelvic pain disorder. IBD, on the other hand, encompasses to inflammatory conditions called Crohn's disease and ulcerative colitis. These are autoimmune conditions where the body's immune system attacks the intestinal mucosal lining, resulting in a variety of symptoms including diarrhea, fatigue, weakness, weight loss, and bleeding. One last inflammatory condition that can be grouped here is pancreatitis, which is a very painful inflammation of the pancreas and is typically associated with gallstones or alcoholism as explained by the National Pancreas Foundation.
Chronic Diarrheal Conditions
According to the CDC, there are several causes of chronic diarrhea that can produce these quartet of symptoms in addition to infectious and inflammatory causes. Chronic disorders of the pancreas can be included here, such as deficiencies in pancreatic enzymes and chronic pancreatitis. Chronic use of medications such as laxatives and antibiotics can produce diarrheal syndromes that result in significant weight loss, fatigue, and weakness. Hyperthyroidism can lead to the aforementioned symptoms, but adds heart palpitations and anxiety. Serotonin-producing tumors such as the case in carcinoid syndromes also lead to these issues, in addition to heart failure and shortness of breath.
The most profound of the conditions that cause these symptoms are described by the Immune Deficiency Foundation as severe combined immune deficiency and combined immune deficiency. These are fatal immune deficiency conditions caused by at least 13 different genetical mutations and most often occur in young boys. These children are susceptible to a multitude of infections and die at an early age. The most common symptom is a persistent and debilitating diarrhea that results in their failure to thrive and grow. Another less serious condition, resulting from genetical mutation is selective IgA deficiency. Sufferers with this condition typically live into adulthood and lead normal lives except for weakness, fatigue and weight loss from recurrent diarrheal syndromes, continual upper respiratory conditions, and serious allergic reactions to blood transfusions. Cystic Fibrosis can also be included in the hereditary causes of these symptoms, but it also has severe respiratory issues and leads to early death.