The above results suggested that aged mice were more susceptible to clinical disease following intranasal HeV exposure than juvenile animals. Using logistic regression analysis to determine if this trend was significant we found that 80% of aged animals developed clinical disease (s.e. = 12.7%) and only 10% of juvenile animals (s.e. = 9.5%) suggesting a real difference (70%, s.e. = 15.8%) in susceptibility to the development of disease between the aged and juvenile animals (p,0.01).