Haemophilus influenzae type b (Hib) is a bacterium that can cause severe, sometimes life‑threatening infections, especially in young children, but modern vaccination has made these diseases much less common in many countries. It mainly spreads via respiratory droplets and can invade the bloodstream and nervous system, leading to conditions such as meningitis and pneumonia.

What Hib Is

Hib is a gram‑negative coccobacillus with a characteristic polysaccharide capsule called polyribosyl ribitol phosphate (PRP), which is a major factor in its ability to cause invasive disease. Among the six encapsulated serotypes (a–f), type b historically caused the vast majority of serious infections in children and a substantial share in adults.

How It Spreads

Hib resides in the upper respiratory tract and is transmitted via droplets when an infected person coughs or sneezes, or through close contact with contaminated secretions. The incubation period is typically a few days, with estimates ranging from about 2 to 10 days before symptoms appear.

Diseases Hib Can Cause

When Hib becomes invasive (enters the bloodstream or sterile sites), it can cause several serious syndromes.

  • Meningitis with fever, lethargy, irritability, vomiting, and risk of rapid neurological decline and long‑term neurological problems such as permanent hearing loss.
  • Epiglottitis with high fever, sore throat, stridor, difficulty breathing, drooling, and risk of sudden airway obstruction and death without prompt airway management.
  • Pneumonia, bacteremia, septic arthritis, cellulitis (especially of the face), and less common complications such as pericarditis, osteomyelitis, or endocarditis.

Who Is Most At Risk

Before widespread vaccination, Hib was a leading cause of invasive bacterial disease in children under 5 years, often responsible for more than 80% of severe H. influenzae infections in this age group. Today, the highest risk remains in unimmunized or under‑immunized young children, as well as some immunocompromised individuals and, in certain settings, unvaccinated adults.

Prevention and Vaccination

Conjugate Hib vaccines that target the PRP capsule have dramatically reduced rates of invasive Hib disease where they are part of routine childhood immunization programs. In many countries, these vaccines are given in infancy as part of combination schedules (for example at 2, 4, 6 months with a booster later), leading to marked declines in meningitis, epiglottitis, and other invasive Hib infections.

Information gathered from public forums or data available on the internet and portrayed here.