With our rapidly ageing population and advancing treatments for patients with haematological, oncologic and rheumatological diseases, there are increasing numbers of immunocompromised patients presenting to primary care and general hospitals with opportunistic infections. This review considers the trends of these infections across four representative subgroups: fungal infections following haematopoietic stem cell transplant; viral infections post solid organ transplant; mycobacterial infections during treatment with targeted biological agents; and bacterial infections as a cause of fever in neutropenia. We also consider the impact of host, pathogens, environments and treatments on the epidemiology and outcomes of these infections.