Smelly vacuum causes...Frito feet!
Smelly vacuums usually come from people who have pets. You per may not shed or be perfectly clean but your vacuum will still get that pet odor and you might wonder why...well here it is. Animals have strong scent glands in their feet and these feet touch the floor and carpet. These glands are designed to leave the scent behind on the ground as they walk so they can find their way back home or smell another animals path. If you have a dog, smell the bottom of their feet...I bet you’ll say they smell like the vacuum bag! Kinda like Fritos corn chips! So we vacuum the floors and carpet and pick this scent up in the dust and dirt on the floor and it goes into the vacuum bag or filter and stays there until we either change the bag or wash the filter and bin. And when the vacuum has a fan the dirt has to go through it too will get the odor on it. A fabric outer bag on a vacuum like a Kirby will obtain them odor as well.
Anyway I’ve found the bagless vacs have less of an issue with this and washing the primary filter after each use keeps it from smelling bad and usually is pretty easy, not much harder than washing a kitchen sponge.