part of a pathogen that can enter the host and can cause disease.

Merriam-Webster Online Dictionary
inoculum (noun)
material used for - inoculation
inoculum (Wikipedia)

The terms inoculation, vaccination, and immunization are often used synonymously to refer to artificial induction of immunity against various infectious diseases. However, there are some important historical and current differences. In English medicine, inoculation referred only to the practice of variolation until the very early 1800s. When Edward Jenner introduced smallpox vaccine in 1798, this was initially called cowpox inoculation or vaccine inoculation. Soon, to avoid confusion, smallpox inoculation continued to be referred to as variolation (from Variola = smallpox) and cowpox inoculation was referred to as vaccination (from Jenner's use of variolae vaccinae = smallpox of the cow). Then, in 1891, Louis Pasteur proposed that the terms vaccine and vaccination should be extended to include the new protective procedures being developed. Immunization refers to the use of all vaccines but also extends to the use of antitoxin, which contains preformed antibody such as to diphtheria or tetanus exotoxins. Inoculation is now more or less synonymous in nontechnical usage with injection and the like, and questions along the lines of "Have you had your flu injection/vaccination/inoculation/immunization?" should not cause confusion. The focus is on what is being given and why, not the literal meaning of the technique used.[citation needed]

Inoculation also has a specific meaning for procedures done in vitro. These include the transfer of microorganisms into and from laboratory apparatus such as test tubes and petri dishes in research and diagnostic laboratories, and also in commercial applications such as brewing, baking, oenology (wine making), and the production of antibiotics.[citation needed] In almost all cases the material inoculated is called the Inoculum, or less commonly the inoculant, although the term culture is also used for work done in vitro.[citation needed]

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