"The problem with such diseases as influenza and HIV is that the viruses have an uncanny ability to change their makeup. Thus, the immune system is not prepared to combat the new or mutant strain. For instance, at intervals the flu virus attacks with a virulent new strain – 1900, 1918, 1957, 1968, 1977."
Russell, Richard. Dow Theory Letters. July 20, 1994. Issue 1152. Page 5.
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