plural infectivities
Definition of infectivity
: the ability to produce or transmit infection : the quality or state of being infective
The onset of cough on the second day of illness in this patient, followed by clinical evidence of pneumonia four days later, raised the question of his infectivity during the four days before hospitalization.— B. Johnson et al. Even when there are apparently high levels of the virus in the blood, there may be much less infectivity. … a thousand or more of these particles are needed to produce infection.— Charles Marwick
specifically
: the capacity to spread from host to host
The swine flu's infectivity is rated at about 2.5, meaning that every infected person can spread the infection to 2.5 people and it doesn't take a math major to understand that implication. — Dennis Bloomfield
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Merriam-Webster unabridged
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Time Traveler for infectivity
The first known use of infectivity was in 1871
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