If you ask „how did people survive“ the answer is more often than not: „with great difficulty“. Take for example simple hygiene measures like using soap or brushing your teeth or disinfecting wounds.

Or, the answer is simply “they didn't survive”, which is the case for countless babies who died because an activity as simple as hand-washing wasn't known to be a matter of life and death.

Teeth brushing is not as important if you're not eating modern food. Processed sugars really are terrible (but delicious)