Effective immunity to MD requires involvement and coordinated
activation of innate defence mechanisms and adaptive immune responses.
The outcome of infection is not only determined by the virulence
of the virus but also by the complex interactions between the
components of both arms of the host immune system involving
cytokines, antibodies, macrophages, natural killer (NK) cells, T helper
(TH) cells, cytotoxic T lymphocytes (CTLs) and soluble factors such
as nitric oxide (NO) (Schat and Markowski-Grimsrud, 2001).