Average life expectancy in the developed world is increasing at a rate of 2 years per decade and, if this continues, by the year 2020 one in five of the population will be aged 65 years and over. Despite the increase in average lifespan, the period of good health enjoyed during a lifetime (healthspan) has not kept pace and there is evidence that the time spent in ill health in old age is increasing (1). Susceptibility to infectious disease increases with age and infection related mortality accounts for almost 15% of deaths amongst those aged over 85 years (1). These data suggest a decline in the functioning of the immune system with age, a process termed immunesenescence. In addition, there are age-related alterations to the Hypothalamic-Pituitary-Adrenal (HPA) axis, a key effector of the response to stress. The reduced ability to produce the immune enhancing hormone dehydroepiandrosterone (DHEA) results may contribute to immunesenescence and leave older adults more susceptible to infections following physical trauma such as hip-fracture (2,3). It is proposed that the age-related changes to both the immune system and the HPA axis result in increased vulnerability to stress in older adults, and factors that can modify the impact of the stress response upon immunity should be sought. One potential intervention is exercise. It is well known that physical activity declines with age in humans and that physically active older adults have fewer infections (4) and evidence will be presented that this is achieved via positive effects on immune system components.