Age, location, and the presence of a viral infection are all inter-related in this situation. At higher altitudes, the pressure of the air around you decreases so there is less oxygen in the surrounding air. Going from a low altitude in Fort Worth, TX (653 ft) to Denver, CO (around 5,300 ft) takes time to get adjusted to. With infection present, that means more time needed to adjust. Elderly have a reduction in providing air to their lungs as is, so when they ascend in altitude they will have an impaired adaptive response. Since the supply of oxygen is lower, the job of getting oxygen to the blood is difficult and with viral infection present, it may be impossible.
