Employer-sponsored
While the majority of Americans -- 59-61 percent -- are insured through their employer, there has been a nearly 10 percent drop in employment-based coverage over the past 20 years. The decline has been attributed to fewer employers offering insurance, a reliance on part-time or contract workers who are not offered insurance, and a decrease in the employee "take-up" rate due to higher co-payments.

Rapidly rising health insurance premiums is the main reason cited by all small firms for not offering coverage. Over the past five years, the average annual increase in health insurance premiums for small firms has been 15 percent, according to the National Coalition on Health Care.

Some companies have tried to share that increase with their employees, which has led to more people declining insurance. Employee spending for health insurance coverage (employee's share of family coverage) has increased 143 percent between 2000 and 2005, according to the National Coalition on Health Care.

In addition, the loss of manufacturing jobs, which traditionally have offered extensive benefits, has pushed workers into the service industries, which generally offer fewer benefits. Studies have found that the movement of workers from the manufacturing sector to the service sector accounts for approximately 10 percent to 15 percent of the decline in employment-based health insurance coverage, according to the Employee Benefit Research Group.

The H5N1 strain of the bird flu is a deadly virus that originated in Southeast Asia. It is found mostly in birds but is known to infect and kill humans in rare instances. Health authorities fear the virus could become easily transmittable from human to human, spark.

'Individual' private insurance
Unlike an employer-sponsored plan that has to accept everyone at the same price, private plans in most states are underwritten based on age, weight, smoking status and health history. Applicants often have to undergo a medical exam and preexisting conditions can increase premiums or even make it impossible to get coverage in some states, such as California and Florida. In places like New York, New Jersey and Vermont, insurers must offer coverage to every applicant, regardless of age or health status, but that has resulted in exorbitant premiums, which can top $20,000 for a family of four. The high premiums, deductibles and co-payments have kept the number of people with private individual insurance down to around 5 percent, with many individuals and their families unable to afford insurance.

Uninsured
Eighteen percent of Americans have no health insurance. Learn more about this group of people here.

Medicaid and other public insurance
Medicaid covers some, but not all, of the low-income and disabled uninsured. Medicaid is larger than any single private health insurer, covering 13 percent of the non-elderly population and over 40 percent of the poor, according to the Kaiser Foundation. It provides health coverage based on both income and categories of eligibility, primarily covering four main groups of non-elderly, low-income people: children, their parents, pregnant women and individuals with disabilities. Medicaid covers one in five people with severe disabilities; however, eligibility in most states is limited to disabled people with incomes below the federal poverty level.

The State Children's Health Insurance Program is designed to cover children who are low-income but whose family incomes are too high to qualify for Medicaid. In 2005, it covered 6 million children.

-- Compiled by Leah Clapman for the Online NewsHour