seroconversion

Short definition from Wiktionary

Noun

1. 1
   Change between the states of seronegativity and seropositivity, in either direction: either the process of this change or an instance of it.
2. 2
   Change between the states of seronegativity and seropositivity, in either direction: either the process of this change or an instance of it: Such change in the negative-to-positive direction, as a result of infection or immunization: the development of specific antibodies in the blood serum.