Haemophilus influenzae is a Gram-negative coccobacillus that occurs in two major forms: encapsulated and non-encapsulated strains. Encapsulated strains are further classified into six serotypes (a–f) based on their polysaccharide capsule, with Haemophilus influenzae type b (Hib) being the most virulent and responsible for over 95% of invasive disease in children. Transmission occurs through respiratory droplets or contact with contaminated surfaces. 

Malaysia incorporated Hib vaccination into the National Immunisation Programme in 2002. Since then, invasive Hib disease among Malaysian children has markedly declined, with only sporadic cases reported. Although primarily a paediatric pathogen, Hib remains clinically important in adults, particularly in older individuals, those with chronic illnesses (e.g., diabetes, COPD), and immunocompromised patients such as those with HIV infection, functional or anatomical asplenia, and transplant recipients. In adults, Hib can cause pneumonia, bacteraemia, and meningitis, all of which are associated with high morbidity and mortality. Hib vaccination is therefore recommended for at-risk adult groups, especially those with asplenia, haematopoietic stem cell transplant, or immunosuppressive therapy, to reduce the risk of invasive disease.