Was it your friend who borrowed your cell phone because the battery in hers was low? Or was it your son's friend who always has a runny nose? How about your annoying colleague who you just KNOW doesn't wash his hands after he blows his nose?
When it comes to getting the flu, few can keep from playing the blame game, especially during a nasty season like this, in which the influenza bug arrived early, spread widely and seems to be staying for a long, miserable visit.
Influenza is now widespread in more than 80 percent of the U.S., according to the Centers for Disease Control and Prevention, with no sign yet of a peak. The most prevalent strain is the A H3N2 virus, experts say, which can cause severe illness. Other viruses besides the flu are also circulating -- CDC data shows only about a third of all people with flu-like symptoms actually are testing positive for influenza.
Even though it's still early in the flu season, hospitals around the country are stretching their resources to face an onslaught of patients sick with the flu. NBC's Dr. Nancy Snyderman reports.
So, at the first sign of the cough, scratchy throat and hit-by-a-truck feeling that flu brings, sufferers can only wonder: Who gave this to me?
Doctors say it's not always possible to pinpoint exactly where you got the flu, though sick friends, family and co-workers are obvious choices, of course.
"It's hard to say 100 percent where you got it, but there are likely culprits," says Dr. Sharon Orrange, an assistant professor of medicine at the Keck Medical Center at the University of Southern California in Los Angeles.
People actually shed the flu virus about a day before they come down with symptoms, so it can be possible for them to infect you before they even know they're sick, said Dr. William Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt University.
But that hasn't stopped people from trying to find the source of their nasty flu infections -- or complaining about them.
In fact, a new Facebook app actually aims at naming friends who might have made you sick. "Help, My Friend Gave Me the Flu," trolls through the posts on your Facebook Page looking for clues that might identify the patient zero who spread your bug.
"It scrubs through your social media contacts looking for 'coughing,' 'sneezing,' 'I feel run down' -- all those keywords," says Richard Fine, chief executive and co-founder of Help Remedies, which sells single-ingredient, over-the-counter drugs that target specific symptoms. He and his crew thought the app would offer a light-hearted diversion to the misery that is influenza.
"When you feel sick, one of the things that we do is look for people to blame," he adds.
In practice, the app appears to implicate folks who tend to post late at night, those who actually say "I have a cold" and spouses, who the app says live "within sneezing distance." It doesn't nab everybody, however. In at least one test, it missed Facebook friends who had previously described their awful flu symptoms in vivid detail. Fine says they're still tuning the algorithm.
In real life, Orrange says that simply knowing someone on Facebook has been sick is no reason to blame that person for your own malady. It's the folks within intimate distance who are most likely responsible.
"If you're at the same table, within five to 10 feet," and the person is sick, you could catch their flu, Orrange says.
More likely, however, you'll pick up their bugs by sharing phones, keyboards, staplers and pens. Or by touching a doorknob that your sick friend or colleague has touched.
There's probably no escaping a sick spouse or child, but it's OK to be annoyed if a colleague comes to work clearly ill, because they actually could infect you, she adds.
"If somebody is actively coughing, with a runny nose, where you actually see nasal discharge, that's bad," she says. "If they are early in their illness and they are right next to you, you are ripe for infection."
The best defense against flu is still vaccination, doctors maintain. This year's vaccines are a good match for this year's viruses, CDC officials say. That doesn't mean the shot guarantees people won't get the flu -- effectiveness in healthy people can range from 65 percent to 80 percent, Schaffner said.
"It's not a perfect vaccine, but it's a pretty good one," he said.
And because the season hasn't reached its peak, there's still a good chance that the vaccine will prevent getting the flu, or at least soften the worst of the symptoms, Orrange adds.
"It's definitely still ramping up," she says. "We feel like we're in the thick of it right now."
By JoNel Aleccia, NBC News