On 17 February 2024, an Airbus A321 Captain was initially unable to re-enter the flight deck after a short absence in the cruise and on succeeding, he discovered that the First Officer had become incapacitated with what ICAO classifies as a ‘seizure disorder’ which was subsequently found to have occurred almost immediately after he had left. Once back on the flight deck, an emergency was declared with immediate diversion to Madrid Barajas where the First Officer was hospitalised. A Safety Recommendation to consider further assessment of the risks of one pilot being alone on a secured flight deck was made.