How do you avoid falling ill on a long train or flight journey?

The realistic levers are modest but real. Choose a window seat where you can — aisle passengers have more contact with people moving about. Ventilation on aircraft is better than most assume because air is filtered and largely vertical, so the bigger risk is the person immediately beside you and the boarding period when air handling is off. On trains, open windows where possible. Bring your own water and avoid cut fruit or food left out. Wash or sanitise hands before eating rather than obsessing over surfaces, and mask if someone near you is visibly unwell.