CHICAGO -- Fran Fleming has been frugal. She and her husband Ed saved to put their three daughters through college, and put money away for themselves.

But even with a pension and Social Security, the 55-year old-Chicago accountant is nervous about retirement.

Fran Fleming. CBS News

"I don't believe I'm comfortable with the amount we've saved today," Fleming says.

That's because Fleming knows medical bills can wreck a family budget. Her late mother's savings dwindled to $11,000 when she developed Alzheimer's and spent her last five years in a nursing home.

"I've seen that the cost of care in a skilled facility is rising faster than salary or savings can possibly rise," Fleming says. "So I need to be prepared for that because none of us want to leave that bill on our kids' shoulders."

High medical expenses were the biggest retirement worry in a new Bankrate.com survey -- 28 percent of respondents ranked it first. Another 23 percent said their top concern was running out of money.

A realistic fear when a couple who retired last year will need an estimated $220,000 to cover future medical costs, according to Fidelity.

"One catastrophic illness or injury can wipe out your savings altogether," says Bankrate's Sheyna Steiner.

Despite that, Steiner says people still aren't putting enough money away.

"Nearly half of people polled said their day-to-day expenses or family obligations are keeping them from saving," Steiner says.

With Americans increasingly uncertain about how much is enough for retirement, Fran Fleming says the message is clear.

"Save more. And I should have done it sooner," Fleming says.

The median value of retirement accounts in the U.S. is now $59,000 -- but less than half of working age Americans have retirement accounts.