How do you stay well when someone at home is already ill?

Assume shared air is the main route rather than surfaces. Open windows and keep air moving, since dilution beats disinfection for respiratory infections. Keep the unwell person in one room where practical, give them their own towel and, if possible, their own bathroom. Wash hands after any contact and before eating. Do not share plates, glasses or pillows. If the ill person can mask when in shared space, that reduces transmission more than anything the rest of the household does. And avoid the instinct to spray every surface — it is low yield for most respiratory bugs.