Epidemiology studies how regularly diseases occur in different groups of people and why – hence its pivotal role in planning the distribution of resources. This chapter focuses on the epidemiology of mental illnesses in older people in acute hospitals and highlights the role of the liaison psychiatrist and other mental health professionals in bridging the gap between physical and mental disorders. By outlining the intricate connection between the two, it explains why careful consideration and a high index of suspicion are needed when assessing older persons in the acute hospital. Emphasis is placed on the importance of viewing ageing as a normal biological and social construct rather than a problematic disease process to which a figurative cut-off point can be applied. The chapter gives prominence to the mammoth social and economic burden of mental disorders, borne by all in society.

The chapter has subsections on important epidemiological terminologies, life expectancy, the prevalence of common psychiatric disorders in acute general hospitals (including emergency departments), and the influence of COVID-19 on the prevalence of mental disorders in older persons. It sounds a clarion call to challenge current practice, work synergistically with all stakeholders, and embrace change if a crisis is to be averted.