3. RISK FACTORS
3.1 The Agent
Neisseria meningitidis (the meningococcus) commonly colonises the nasopharynx without causing disease. Strains associated with invasive disease have acquired certain virulence factors, which are, as yet, poorly understood.
3.2 The Host
Medical conditions that commonly predispose individuals to invasive disease include:
- Deficiencies of the terminal components of the complement system
- Functional or anatomical asplenia
3.3 The Environment
The risk of infection is related to the nature and duration of contact. Household contacts of a case of meningococcal disease have a 400 - 800 fold increased risk of infection compared to the general population. Household overcrowding, coexisting viral infection and especially exposure to tobacco smoke also increase the risk. However despite perceived risk, only 0.5% of cases are associated with a household contact. When fairly large numbers of first year university/ technikon students, military or police recruits live together in residences, hostels or barracks they also have a 3 times higher incidence of disease than in the general population.