5.1 Introduction
The goal of vaccine development is the induction of effective protective immunity. Naturally occurring protective immunity, which can develop after primary exposure to infectious agents and protects against reinfection with the same pathogen,was re- cognized already in ancient Greek times; as early as 430 BC, during the Peloponne- sian War,Thucydides recorded that people who had recovered from the plague could care for infected individuals without getting the disease a second time [1]. With vaccination, the immune system is exposed to antigen in a noninfectious or mildly infectious way,with the goal of protection against subsequent infection.Vacci- nation has been attempted since the 11th century,when the Chinese inoculated chil- dren by inhalation of scab from the smallpox pustules of mildly infected people or by rubbing the dried material into a scratch. However, general acceptance of this technique was understandably limited, due to the high risk of accidentally inducing lethal infection with human smallpox material. Jenner overcame this problem by using a cross-reactive bovine analog of smallpox, vaccinia or cowpox (vacca = cow). This vaccination strategy was so successful that the WHO declared the eradication of smallpox in 1980.
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