Traveling long distances can greatly reduce the immunity of a person and increase the possibility that he or she will fall ill.
For many people, especially those who do not travel regularly, travel can be stressful, since a lot of preparation is required. Many people do not sleep well while traveling because they have to be alert, reducing their immunity.
Also while traveling the person is often in the vehicle with a large number of people who may have some infectious disease. Being exposed to air containing high levels of viruses, or other harmful microbes, can also cause illness.
So it is advisable to reduce travel to the extent possible.