Influenza (Flu) Overview

Every winter, flu infects some 40 million Americans, costing the nation $928 million in medical expenses and $10.5 billion in lost school and work days. The worst cold might lead to bronchitis or a sinus infection. But for the elderly or those with asthma and other chronic respiratory conditions, flu can be fatal, progressing to a form of bacterial pneumonia that kills tens of thousands of Americans every year.

In addition, once or twice each century, a particularly lethal flu strikes. The 1918-19 epidemic killed 20 million people worldwide (700,000 in the U.S). No one knows when the next killer flu will strike, but the experts agree we’re about due.

If you’re like most Americans, you call any bad cold “the flu.” Like the common cold, the flu is an upper respiratory viral infection. But that’s where the similarities end. The flu, short for “influenza,” is often much more severe.

“Most people under 65 still see the flu as no big deal,” says flu specialist Steven Mostow, M.D., chairman of the department of medicine at the University of Colorado Health Sciences Center in Denver. “That’s a big mistake. Even if you’re healthy and in the prime of life, flu can knock you flat on your back for a week. And if you’re elderly or have chronic health problems, it can kill you.”

In the U.S., the annual flu season runs from Thanksgiving to Easter, though the worst outbreaks typically occur from late December to early March.

Flu is caused by three viruses, identified as A, B, and C. Type-A flu (also called influenza A) causes the worst symptoms and the most complications, says Nancy Arden, R.N, a senior official at the Influenza Branch of the Centers for Disease Control and Prevention (CDC) in Atlanta. Type-B flu can also cause severe illness, but it’s symptoms tend to be less severe and don’t linger as long. Type C flu is hardly even an illness. When public health officials say “flu,” they mean Type A and B.

Then there’s “stomach flu.” “It’s not flu at all,” Dr. Mostow explains, “It’s a common misnomer for viral infections of the digestive tract, medically known as gastroenteritis.”

Flu spreads through the air. You inhale the virus Colds can also spread through the air, but flu spreads much more easily. “If you put a cold sufferer in a room full of people who are susceptible, several will catch the cold,” the CDC’s Arden says. “But if you put a person with influenza A in that same room, within one to three days, most of them will get the flu.”

One famous study of flu transmission involved 54 passengers on a commercial jet. One boarded the five-hour flight infected with influenza A, and within three days, 72 percent of the other passengers came down with flu. Another involved a tour group traveling to Alaska. One had influenza A, and within a few days, more than half the group had caught it.