Co-founder of GammaDelta Therapeutics and Head of Division of Immunology, Infection & Inflammatory Diseases at King's College London. He also co-founded biotech company ImmunoQure, and was a member of MedImmune’s Science Advisory Board. Until 2019, he was also the lead-investigator of a Wellcome Trust-supported, multi-centre, high-throughput phenotypic screening programme to identify novel genetic regulators of the immune system.

Hayday began studying immunology in 1982 at MIT. He completed Bachelor's in Biochemistry at the University of Cambridge and obtained a PhD in Molecular Virology from Imperial College London.