Time Magazine published an article about "health-sharing ministries," in which Christian families pool their resources to pay one another's medical costs. Although the article discusses the difference between this system and "traditional" insurance, this system is insurance at its most basic.
Insurance is nothing other than a bunch of people getting together to pool their risk. Everyone puts in a set amount of money, perhaps based on the actual risk to the individual participants or perhaps a flat fee. When the thing insured against happens -- a fire, a health event, a car accident -- then the participant to whom it happened receives money from the pool to cover their loss.
Things are complicated by the fact that insurance companies are often multinational corporations that make or lose a lot of their money from investments funded by the premiums. But that does not make their basic function -- pooling risk -- any different.