Haemophilus influenzae bacteria. Coloured transmission electron micrograph (TEM) of Haemophilus influenzae b (Hib) bacteria (red) in vitrified water (pink). These are Gram-negative, non-motile, rod-shaped bacteria that are typically found in pairs (diplococcus). Strain b is the most likely to cause infection in humans. Hib is often found in the respiratory tract of healthy individuals causing no disease. However, in the very young or those with a weakened immune system, it can cause a number of diseases, including meningitis, cellulitis and bacterial pneumonia, which can be fatal.