Electon microcope image of the influenza virus. Image from Wikipedia . Prior to 1933, the flu was thought to be caused by the bacteria Haemophilus influenzae and not a virus. Since 1933, we now know that the flu is actually caused by the influenza virus (types A, B, and C). However, there is much confusion among patients and even healthcare professionals who still erroneously consider Haemophilus influenzae and the flu or influenza to all be the same thing... they absolutely are not! …