Misplaced keys, a forgotten name, needing an extra moment to remember where you parked your car. Most likely we’re talking run-of-the-mill absentmindedness or just normal age-related changes, not Alzheimer’s disease. But when you begin struggling with tasks that have always been easy — e.g., suddenly not being able to balance the checkbook — that’s more cause for concern, says Nancy Udelson, the executive director of the Alzheimer’s Association’s Cleveland Area Chapter.

Other red flags are new problems with words, speaking or writing; losing track of dates, seasons or the passage of time; suddenly using poor judgment — such as forgetting to bathe — and withdrawing from normal social activities.

Despite the fact that 5.1 million people age 65 and older are currently affected by Alzheimer’s, it is not a “normal” part of aging (though the risk does rise with age). It is a progressive disorder that attacks the brain’s nerve cells, explains Carol Steinberg, the executive vice president of the Alzheimer’s Foundation of America. The brains of people who have Alzheimer’s are clogged with lesions that form around the outside of neurons, ultimately destroying these brain cells.

Wow Fact

Those who both exercise regularly and follow a Mediterranean-style diet are 59 percent less likely to develop the symptoms of Alzheimer’s, says a Columbia University study, than those with a couch-potato lifestyle and high-fat eating habits.

As the baby boomer population gets older, the incidence of Alzheimer’s is projected to explode, affecting as many as 16 million people by 2050, unless medical breakthroughs identify ways to prevent or more effectively treat the disease.

If you have family members who have suffered from Alzheimer’s, you may wonder if you’re at higher risk, but only the “early onset” form of the disease (which affects people younger than 65 and accounts for less than 5 percent of all cases) seems to have a genetic link. Something else to put your mind at ease: It’s possible to develop the lesions in the brain typical of Alzheimer’s but never display the symptoms.

The secret to outsmarting this disease? Building up a reserve of brain cells through mentally challenging activities, following a healthy diet and exercising regularly.

QUIZ: How Active Are You?