The more conspicuous characteristics of the blood in various diseases have been mentioned in previous sections. Although the great majority of blood changes are secondary, there are a few blood conditions in which the changes are so prominent, or the etiology so obscure, that they are commonly regarded as blood diseases. These will receive brief consideration here. A. ANEMIA This is a deficiency of hemoglobin, or red corpuscles, or both. It is either primary or secondary. The distinction is based chiefly upon etiology, although each type presents a more or less distinctive blood-picture. Secondary anemia is that which is symptomatic of some other {204} pathologic condition. Primary anemia is that which progresses without apparent cause. 1. Secondary Anemia.--The more important conditions which produce secondary or symptomatic anemia are: (_a_) _Poor nutrition_, which usually accompanies unsanitary conditions, poor and insufficient food, etc. (_b_) _Acute infectious diseases_, especially rheumatism and typhoid fever. The anemia is more conspicuous during convalescence. (_c_) _Chronic infectious diseases_: tuberculosis, malaria, syphilis, leprosy. (_d_) _Chronic exhausting diseases_, as heart disease, chronic nephritis, cirrhosis of the liver, and gastro-intestinal diseases, especially when associated with atrophy of gastric and duodenal glands. The last may give an extreme anemia, indistinguishable from pernicious anemia. (_e_) _Chronic poisoning_, as from lead, arsenic, and phosphorus. (_f_) _Hemorrhage_--either repeated small hemorrhages, as from gastric cancer and ulcer, uterine fibroids, etc., or a single large one. (_g_) _Malignant tumors_: these affect the blood partly through repeated small hemorrhages, partly through toxic products, and partly through interference with nutrition. (_h_) _Animal Parasites_.--Some cause no appreciable change in the blood; others, like the _Uncinaria_ and _Bothriocephalus latus_, may produce a very severe anemia, almost identical with pernicious anemia. Anemia in these cases is probably due both to toxins and to abstraction of blood. The blood-picture varies with the grade of anemia. {205} Diminution of hemoglobin is the most characteristic feature. In mild cases it is slight, and is the only blood change to be noted. In very severe cases hemoglobin may fall to 15 per cent. Red corpuscles are diminished in all but very mild cases, while in the severest cases the red corpuscle count is sometimes below 2,000,000. The color index is usually decreased. Although the number of leukocytes bears no relation to the anemia, leukocytosis is common, being due to the same cause.