An innovative AI tool named Delphi-2M, developed using generative large language model architectures, can forecast the likelihood and timing of developing 1,258 diseases up to 20 years in advance. Trained on 400,000 UK Biobank participant health records incorporating demographics and lifestyle factors, the model offers an unprecedented comprehensive approach beyond single-disease predictions. Such predictive capacity could transform preventative medicine by enabling early interventions targeting high-risk individuals.