A form of malnutrition (Protein-energy malnutrition, to be exact), occurring particularly in young children (under five years of age). While marasmus does occur in adults, it is more often a problem with young children, and so I will focus on its occurrence in children in this writeup. Young children have increased energy needs, to fuel their growth. They are more susceptible to infectious diseases, which can encourage malnutrition, and they are also less able to get food for themselves, and may be fed less, or less desirable, food then are adults. (It is worth noting that most marasmus-related deaths in America {there aren't many} occur in the elderly).
The definition of marasmus is not always agreed on; it is very close to kwashiorkor in both cause and symptoms. One common distinction between the two is that marasmus is caused by an inadequate intake of both protein and calories, while kwashiorkor is caused by inadequate intake of protein but adequate (or at least not starvation level) caloric intake. This doesn't seem to be quite enough to explain the occurrence of one rather than the other, but it is a good start. As far as the symptoms go, they can be distinguished on a simple level by noting the presence (in kwashiorkor) or lack (in marasmus) of edema (children with marasmus may show some signs of edema, but it is not as severe is in children with kwashiorkor).
One theory is that marasmus is a 'successful' adaptation to near-starvation, while kwashiorkor is a failure to adapt. Needless to say, neither of these disease are healthy; marasmus is simply the healthier of the two.
The symptoms of marasmus vary, as it is usually accompanied by various vitamin deficiencies. In addition, marasmus is often caused by other diseases that inhibit digestion, most notably diarrhea (diarrhea is a big killer, worldwide; aside from preventing intake of nutrients, it leads to dehydration); it may be hard to distinguish the symptoms of these other diseases from that of marasmus.
The classic symptoms include:
The weight loss is due to both fat and muscle loss, as the body salvages what protein, amino acids, and energy it can. Fat is first taken from the from the legs and upper body; the face, particularly the cheeks, are the last to be affected. The apathy and muscle loss are in part an attempt to save energy; the apathy may also be a result of potassium deficiency, which usually accompanies marasmus. Any number of nutritional deficiencies can accompany marasmus; iron deficiency anemia is common. As you might expect, the fat-soluble vitamins are often lacking.
A child with marasmus is weakened, and therefore more vulnerable to almost any type of diseases. Due to the lowered metabolic rate, they are at more risk from hypothermia, and are less likely to have a fever if infected (remember, fever is meant to do you good, as long as it doesn't get out of hand.) The detoxifying function of the liver is impaired; the immune system is impaired (specifically, T-lymphocyte producing tissues are impaired; IgA production is impaired; mucosal integrity and lymphokine production are impaired); heart function is impaired, and bradycardia and hypotension are common in severe cases. In severe cases brain function and cognitive development may be impaired, although the body does a good job of protecting the brain for as long as possible.
When recovering, energy needs are above-average when compared to other children of the same age, as the body tries to put on weight and make up for lost growth. In severe cases, many small meals are needed to prevent death from hypoglycemia (the reason for hypoglycemia in recovering marasmitic children is not well understood). The child may have trouble absorbing fats during early renutrition (yes, that is a real word. It means recovering from malnutrition). In many cases a full recovery is possible, particularly if appropriate medical care is available.
The causes of marasmus aren't simply lack of food; as mentioned above, diarrhea is an important cause of marasmus. Diarrhea doesn't come out of nowhere. One of the leading causes of marasmus is the weaning of a child. Mother's milk helps protect a child from harmful bacteria, both in that it is sanitary and in that it contains 'good' bacteria which inhabit the intestinal tract, preventing 'bad' bacteria from gaining a hold.
In the past, the aggressive marketing of baby formula into third world countries has been an important cause of diarrhea, and therefore marasmus, both because it meant the baby wasn't drinking the mother's milk, and because the mothers didn't understand the necessity of boiling the water, or couldn't afford to feed their babies pure formula, diluting it with tea, milk, coffee, and other things baby's stomachs aren't suited for. Recent public health programs have made some headway in keeping mothers from ending breastfeeding before the infant's immune system is ready to fend for itself. Improved sanitation and health education are also helping. The number of mothers entering the workforce in third world countries is still a growing problem, as it encourages early weaning.