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- Humans have evolved numerous mechanisms to respond to physical, biotic and social threats.
- The major causes of extrinsic mortality are biotic, and include competition for nutrient supplies with other organisms of the same or different species; predation; and infection by micro-organisms.
- Detection of a stressor elicits physical, endocrine and behavioural responses that evolved to cope with the threat or perceived threat. There may be pathological consequences of inappropriate stress responses.
- Pathogens and parasites have co-evolved with their hosts.
- The adaptive immune system is a key component of protection against microbial infection in vertebrates. Its specificity is a consequence of variation and selection.
- Variations in drug-metabolising ability may be an echo of past adaptive responses to new food sources encountered by migrating humans.
Age-specific mortality rates in the influenza epidemics of 1911-1917 and the ‘Spanish’ influenza epidemic of 1918
The high virulence of the 1918 virus may have resulted from an aberrant host immune response that caused extensive tissue damage in the lungs; in addition, previous exposure of older persons to an antigenically related virus several decades earlier may have contributed to the relatively higher mortality in younger people.