Usually, the influenza virus changes its structure very slightly from one year to the next. However, although we are being exposed to these “modified” influenza strains every year, we do not always come down with the flu, even when the virus successfully breaches physical barriers. However, some years we do get a really bad case of the flu, despite the fact that we presumably have memory cells left from an earlier primary response to influenza (via vaccine or natural acquisition). Aside from higher doses of virus and the possibility of a particularly pathogenic strain, why is it that some years we get very sick and other years we do not? For example, I might get a bad case of the flu while you experience no disease, and yet we are both being exposed to the exact same virus strain that year. What is happening here? You can assume that you are not receiving the yearly influenza vaccine.