Professor Macallan is a specialist in adult infectious diseases. His special interests include HIV infection and tuberculosis. He has a regular HIV outpatient clinic at St George's University Hospitals NHS Foundation Trust, as well as a specialist clinic for patients who have both HIV and tuberculosis.

Professor Macallan qualified from the University of Oxford and the London Hospital Medical College in 1983. He completed a PhD on the nutritional and metabolic complications of chronic infection at St George's medical school in 1994.

He went on to train in general internal medicine and infectious diseases in London and is on the specialist register for both specialties. He completed a diploma in tropical medicine and hygiene in 1998.

Professor Macallan has been an honorary consultant physician at St George's University Hospitals NHS Foundation Trust since 1998 and was made a professor at St George's, University of London in 2007.