The onset of sickness close to the start of a holiday might have nothing to do with travelling to an unfamiliar place. When I was teaching, every teacher I knew assumed that they'd either fall ill, or at least feel unwell, during the first few days of each school holiday. That was irrespective of whether they were taking a holiday away from home or simply putting their feet up in their favourite armchair. It seemed to have something to do with 'keeping going', under stress' until the end of term and then, as the stress was finally removed, the body's immune system deciding to take a break too.