The fundamental concept of insurance is that it balances costs across a large, random sample of individuals (see risk pool). For instance, an insurance company has a pool of 1000 randomly selected subscribers, each paying $100 per month. One person becomes very ill while the others stay healthy, allowing the insurance company to use the money paid by the healthy people to pay for the treatment costs of the sick person. However, when the pool is self-selecting rather than random, as is the case with individuals seeking to purchase health insurance directly, adverse selection or adverse selection insurance is a greater concern.[13] A disproportionate share of health care spending is attributable to individuals with high health care costs. In the U.S. the 1% of the population with the highest spending accounted for 27% of aggregate health care spending in 1996. The highest-spending 5% of the population accounted for more than half of all spending. These patterns were stable through the 1970s and 1980s, and some data suggest that they may have been typical of the mid-to-early 20th century as well.[14][15] A few individuals have extremely high medical expenses, in extreme cases totaling a half million dollars or more.[16] Adverse selection could leave an insurance company with primarily sick subscribers and no way to balance out the cost of their medical expenses with a large number of healthy subscribers.

Because of adverse selection, insurance companies employ medical underwriting, using a patient’s medical history to screen out those whose pre-existing medical conditions pose too great a risk for the risk pool. Before buying health insurance, a person typically fills out a comprehensive medical history form that asks whether the person smokes, how much the person weighs, whether the person has been treated for any of a long list of diseases and so on.  In general, those who present large financial burdens are denied coverage or charged high premiums to compensate.[17] One large U.S. industry survey found that roughly 13 percent of applicants for comprehensive, individually purchased health insurance who went through the medical underwriting in 2004 were denied coverage. Declination rates increased significantly with age, rising from 5 percent for individuals 18 and under to just under a third for individuals aged 60 to 64.[18] Among those who were offered coverage, the study found that 76% received offers at standard premium rates, and 22% were offered higher rates.[19] On the other side, applicants can get discounts if they do not smoke and are healthy. That kind of insurance is so call adverse selection insurance because they try to screen out the adverse selection.

A health insurance policy is a contract between an health insurance company and an individual or his sponsor (e.g. an employer). The contract can be renewable annually or monthly. The type and amount of health care costs that will be covered by the health insurance company are specified in advance, in the member contract or “Evidence of Coverage” booklet. The individual insurered person’s obligations may take several forms.

* Premium: The amount the policy-holder or his sponsor (e.g. an employer) pays to the health plan each month to purchase health insurance coverage.
* Deductible: The amount that the insured must pay out-of-pocket before the health insurer pays its share. For example, a policy-holder might have to pay a $500 deductible per year, before any of their health care is covered by the health insurer. It may take several doctor’s visits or prescription refills before the insured person reaches the deductible and the insurance company starts to pay for care.
* Copayment: The amount that the insured person must pay out of pocket before the health insurer pays for a particular visit or service. For example, an insured person might pay a $45 copayment for a doctor’s visit, or to obtain a prescription. A copayment must be paid each time a particular service is obtained.
* Coinsurance: Instead of, or in addition to, paying a fixed amount up front (a copayment), the co-insurance is a percentage of the total cost that insured person may also pay. For example, the member might have to pay 20% of the cost of a surgery over and above a co-payment, while the insurance company pays the other 80%. If there is an upper limit on coinsurance, the policy-holder could end up owing very little, or a great deal, depending on the actual costs of the services they obtain.
* Exclusions: Not all services are covered. The insured person is generally expected to pay the full cost of non-covered services out of their own pocket.
* Coverage limits: Some health insurance policies only pay for health care up to a certain dollar amount. The insured person may be expected to pay any charges in excess of the health plan’s maximum payment for a specific service. In addition, some insurance company schemes have annual or lifetime coverage maximums. In these cases, the health plan will stop payment when they reach the benefit maximum, and the policy-holder must pay all remaining costs.
* Out-of-pocket maximums: Similar to coverage limits, except that in this case, the insured person’s payment obligation ends when they reach the out-of-pocket maximum, and the health company pays all further covered costs. Out-of-pocket maximums can be limited to a specific benefit category (such as prescription drugs) or can apply to all coverage provided during a specific benefit year.
* Capitation: An amount paid by an insurer to a health care provider, for which the provider agrees to treat all members of the insurer.
* In-Network Provider: (U.S. term) A health care provider on a list of providers preselected by the insurer. The insurer will offer discounted coinsurance or copayments, or additional benefits, to a plan member to see an in-network provider. Generally, providers in network are providers who have a contract with the insurer to accept rates further discounted from the “usual and customary” charges the insurer pays to out-of-network providers.
* Prior Authorization: A certification or authorization that an insurer provides prior to medical service occurring. Obtaining an authorization means that the insurer is obligated to pay for the service, assume it matches what was authorized. Many smaller, routine services do not require authorization.[9]
* Explanation of Benefits: A document sent by an insurer to a patient explaining what was covered for a medical service, and how they arrived at the payment amount and patient responsibility amount.[10]

Prescription drug plans are a form of insurance offered through some employer benefit plans in the U.S., where the patient pays a copayment and the prescription drug insurance part or all of the balance for drugs covered in the formulary of the plan.

The second part of how health insurance works is after you use health care providers’ services. Some, if not most, health care providers in the United States will agree to bill the health insurance company if patients are willing to sign an agreement that they will be responsible for the amount that the health insurance company doesn’t pay. The health insurance company pays out of network providers according to “reasonable and customary” charges, which may be less than the provider’s usual fee. The provider may also have a separate contract with the insurer to accept what amounts to a discounted rate or capitation to the provider’s standard charges. It generally costs the patient less to use an in-network provider.