The immune system provides a strong response to even a low exposure to virus.

How influenza virus dose affects the size of the immune response has not been clearly documented. Mice were challenged with three doses of influenza virus spanning a 100-fold range. Increasing the viral input dose increased the degree of weight loss observed, the clinical score and eventual mortality. Maximum viral loads increased with viral input and lower doses peaked and declined earlier. The level of the immune response only varied 2-fold and was independent of viral dose with near maximal responses elicited by the lowest dose, as measured by influx of antigen-specific and non-specific leukocytes into the lungs and by influenza antibody titers. We conclude that a strong immune response is mounted to a small dose of virus and curbs the spread of virus early and prevents weight loss whereas larger doses of virus elicit a slightly greater response but the associated disease can overwhelm the host.