Chlorosis in plants is when the leaves turn from green to yellow. This is due to a lack of chlorophyll, the green pigment in leaves, which is in turn caused by the plant not being able to get enough iron. The plant may not be able to get enough iron if its roots are damaged, if the soil is waterlogged, the soil is low in iron, or if the soil is very alkaline, which causes the iron to change into a less-easily-absorbed form.
There are four basic ways to correct this. One is to spray the affected plant with an iron solution. This provides a quick fix, but does not solve the problem, and only affects the parts directly sprayed. Another is to introduce more iron into the soil; the drawback with this is, if the plant roots are damaged, the plant will not be able to absorb it anyway. If the problem is the high pH of the soil, it can be lowered, but this will not help if any of the other factors are a problem. Lastly, iron salts can be injected directly in to the main stem of the plant.
Chlorosis in humans was a disease diagnosed among young female city-dwellers from Renaissance to Victorian times. It is no longer diagnosed, and we can only have theories about what it actually was. It was also known as the green sickness and the Virgin's disease.
The symptoms were pale skin, irregular menstruation, breathlessness and a weak heartbeat. Basically, these girls were sickly. They also lost their appetites, particularly for red meat, but sometimes ate non-food objects (pica).
There are quite a few possible causes for these symptoms. One suggestion is that it was not a disease, per se, but the cummulative effect of the conditions in which the girls lived. They would have worn corsets, leaving them feeling weak and breathless, and possibly eating would have been uncomfortable. Their diet would have been poor, leading to vitamin and nutrient deficiencies. The shortness of breath could have been asthma brought on by factory conditions or tuberculosis, which was very common at the time.
Another suggestion, but one which could be a cause or a symptom, is iron-deficiency anaemia. Patients went off red meat specifically, as well as generally losing their appetites. As red meat is a major source of iron, this could be an explanation for lethargy, paleness and shortness of breath. It also fits with the age and gender of the victims, as menstruating women need more iron to compensate for blood loss. (This also provides a pleasing link with chlorosis in plants, but I just like spotting patterns.)
Some of the patients could have been anorexic, if the apparent loss of appetite was the actual illness, rather than just being one of the symptoms. The symptoms of chlorosis - paleness, weak heartbeats and irregular menstruation - are all caused by malnutrition, and are present in advanced anorexia nervosa. Of course, the patients could be starving involuntarily with similar effects.
Sources: 'Chlorosis: the Virgin's Disease' by Margaret Humphries, and a bunch of websites that told me more than I wanted to know about plants, including http://www.oznet.ksu.edu/dp_hfrr/extensn/problems/irnchlor.htm, http://www.ianr.unl.edu/pubs/plantdisease/g1218.htm, http://www.urbanext.uiuc.edu/focus/chlorosis.html