Infection and immunity
Infection
Infectious (transmissible) disease
Organism
Bugs
Host
Clinical features
Carrier
Immunity

Probability of exposure leading to infection
Virulence of organism
Number of organisms
Immune defences

Humans and bacteria
Skin, mouth and vagina (mucous membranes), GI tract
Internal tissues are sterile

Relationships
Commensalism, mutualism or parasitic relationship
Commensalism – organisms which live with us but normally cause no harm, (commensal flora)
Mutual bacteria derive benefit from us and we from them, e.g. some coliforms produce vitamin K
Parasitic relationship the infecting organism benefits to the host’s detriment

Pathogenic microorganisms
Pathogens - virulent bacteria or viruses which usually cause disease if present
Commensal organisms can become pathogenic, opportunistic pathogens

Opportunistic microorganisms
Often of low pathogenicity but may cause infections in certain circumstances
Reduced host defences

Local and systemic infection
Inflammation
Conjunctivitis
Boils
Abscess
Osteomyelitis
Cellulitis
Balanitis
Systemic

Epidemiology
Causes, incidence, distribution and control of disease
Endemic
Epidemic
Pandemic
Mortality rate
Morbidity
Incidence
Prevalence