Summary

• The body is continually exposed to infectious organisms that have the potential to cause disease (pathogens). Most pathogens are prevented from entering the body by a combination of physical, chemical and biochemical defence mechanisms. However, some pathogens can breach the barriers and in some cases the barriers are breached by injury or other causes.
• Pathogens vary enormously in terms of size, ways in which they enter the body, how they multiply, whether they replicate intra- or extracellularly, replication rates, mechanisms by which they spread through the body and ways in which they actually cause disease.
• The variety of pathogenic lifestyles means that the immune system must have an equally varied repertoire of mechanisms for dealing with the diversity of threats.