After obtaining a medical degree from the University of Athens and training in internal medicine in Boston, Dionysios Neofytos specialised in infectious diseases at Thomas Jefferson University Hospital in Philadelphia. In 2008, he obtained a Master of Public Health (MPH) from Thomas Jefferson University in Philadelphia. Until 2014, he worked in the division of infectious diseases as an infectious diseases specialist in the field of infectious disease complications in immunocompromised hosts at Thomas Jefferson University Hospital in Philadelphia, Johns Hopkins University School of Medicine in Baltimore and Memorial-Sloan Kettering Cancer Center in New York. 

In 2016, he joined the Infectious Diseases Department at the HUG. At the UNIGE's Faculty of Medicine, he was appointed as a privat-docent in 2021, a lecturer in 2023, then associate professor in the Department of Medicine in September 2025. A specialist of complications of infectious diseases in patients with haematological malignancies and in recipients of bone marrow and solid organ transplants, his research focuses on the epidemiology, diagnosis and management of invasive viral and fungal infections in these patients.

He is also a member of several national and international scientific societies, including the Board of the Swiss Infectious Diseases Society (SSI), Chair of the SSI Guidelines Committee, President of FUNGINOS (Fungal Network of Switzerland), and a member of the Board and Scientific Committee of the Swiss Transplant Cohort Study (STCS).  He is also a member of the EBMT (European Bone Marrow Transplant), ECIL (European Conference for Infections in Leukemia), and the European Society of Clinical Microbiology and Infectious Diseases.