It's pretty common to clean your sheets once a week, and you throw on your pillowcases once. Afterall you spend plenty of amount of time in your bed and you also need to earn certain that it is clean. But often, the pillow by itself has neglected. It seems odd an object near the thoughts for hours at some time will not get washed.
https://www.pillowbedding.com/how-to-wash-throw-pillows/