When immunity is acquired as the result of an infection it is said to have been acquired naturally (a very different thing from natural immunity). Immunity which is acquired artificially may be active, as in vaccination; or passive, as when diphtheria antitoxin is injected prophylactically.
Acquired Active Immunity.
As an illustration of naturally acquired active immunity, a person who has recovered from scarlet fever, smallpox, plague or typhoid fever, becomes possessed of prolonged protection against subsequent attacks. This is also true of many animal diseases. On the other hand, the protection afforded by an attack of certain other diseases is of shorter duration: cholera, diphtheria, pneumonia, etc., at least in many instances. So far as is known, the protection which is acquired is very specific in character, i. e., a person who has had measles may still have scarlet fever, or an attack of
IMMUNITY.CHAPTER VII. ACQUIRED IMMUNITY.. JAMA. 1905;XLIV(10):791–793. doi:10.1001/jama.1905.92500370039002
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