Virulence. Intrinsic virulence of the virus was obviously exceptional. Its effects were compounded further by wartime stress and crowding and secondary bacterial infections.
Absent specific immunity. The reciprocal of antigenic novelty. Young adults would have been primed in infancy with other than HINI viruses (i.e. viruses circulating earlier than 1908), which may explain their unusual susceptibility to fatal outcome with the swine HINI virus.