The acronym HACEK describes a heterogeneous group of organisms that share three major characteristics. First, they are small gram-negative rods that are commonly present as part of normal oral–pharyngeal or respiratory flora. Second, they are relatively fastidious microorganisms. Third, they have a predilection to infect heart valves. The HACEK group includes Haemophilus species (except Haemophilus influenzae), Aggregatibacter (formerly Actinobacillus) species, Cardiobacterium hominis, Eikenella corrodens, and Kingella species. These organisms are infamous for their ability to cause endocarditis although, rarely, they can also cause a variety of other infections (Table 137.1). For example, human bites can result in cellulitis or abscess formation resulting from HACEK organisms, especially Eikenella species, and various Haemophilus species can cause epiglottitis or brain abscesses.