Haemophilus influenzae type b, or Hib, is a bacterium is commonly found in the noses and throats of healthy individuals living in regions of the world where vaccination against it is not carried out.

The bacterium is spread by exhaled droplets. By the age of five, almost all unvaccinated children are exposed to it.

Occasionally, the Hib bacterium in the nose and throat of an infected person can invade their bloodstream, causeing infection and disease in other parts of the body, including the meninges (membranes enveloping the brain and spinal cord) leading to meningitis, and the lungs, causing pneumonia.

Almost all victims of Hib are children under the age of five, with those between four and 18 months of age especially vulnerable.

Hib can cause other life-threatening complications in young children too, such as septic arthritis (an inflammation of the joints) and septicaemia (blood poisoning). It may also lead to epiglottitis, where the flexible cartilage that covers the gap in the vocal cords during swallowing is affected. A condition which can lead to suffocation in extreme cases.

Since routine vaccination for Hib was introduced to many industrialised nations in the 1990s, the incidences of infection has dropped sharply. But in the developing world, Hib is still a danger. It is thought to cause around three million serious illnesses and over 350,000 deaths per year.  However, this is only an estimate as Hib is difficult to diagnose as being the cause of the meningitis or pneumonia. Unlike measles, polio or diphtheria, Hib does not cause uniquely 'Hib' symptoms, which makes it difficult to identify, so Hib could have caused many more deaths than the records show.