Am I contagious? A handy guide for cold and flu season

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We've all been there.

You're at work, or school, or a party, and someone nearby is sneezing and coughing up a lung. Maybe you give him the evil eye; maybe you suggest he go home and get some rest.

His response? Invariably, it's "Oh, I'm not contagious."

That might provide you with little comfort. Just hearing it might inspire a tickle in your throat or febrile warmth to spread across your body.

So here's some chicken soup for the mind in the form of practical advice about when we are - and aren't - contagious.

When it comes to colds and flus, is there a point when I'm definitely contagious?

Yes. Doctors say you're generally contagious from about one day before symptoms begin through the second or third day of your illness.

Dr. Daniel Vigil, a UCLA physician, explains the timeline this way: "It's that little tickle in your throat and you're thinking, 'I hope this isn't a cold or the flu coming on.' That's about the time when the contagious period starts."

He continues: "And then sure enough, you wake up the next day and you feel really crummy – body aches, and you're coughing and sneezing – and about two days after that, which is the third day basically, is the time frame in general that the infected person sheds virus."

When can I say that I'm not contagious anymore?

This is where things get a little more nuanced.

Dr. Jonathan Grein, an infectious disease physician and director for hospital epidemiology at Cedars-Sinai Medical Center, says while you're much less contagious after the third day of a cold or flu, you're not necessarily healthy and home free after that.

After your fever has resolved and your body has mounted a strong immune response against the virus, "your risk of being contagious drops pretty significantly but it may not go to zero for maybe a week or two after you've begun to recover," Grein says.

So when should I definitely stay home sick?

There are at least three scenarios which call for climbing into bed with a good book, rather than taking the risk of spreading your virus to others:

But there's a caveat: Sometimes coughs linger, long after you've recovered from your cold.

Dr. Vigil says he frequently sees patients with this concern. He diagnoses them with what he calls "a post-viral cough."

That, he says, "is a medical, technical way of saying, you got that last little tickle in your throat that is taking its sweet time resolving and healing and going away, but you're certainly not contagious."

What can I do to prevent catching whatever is going around?

That second piece of advice may seem like common sense. But it's important to understand why this prevention strategy works.

When people sneeze and cough, they spread little respiratory droplets that contain a virus. Those droplets contaminate the environment and the things that sick people touch.

So while you're at risk of getting sick if someone coughs directly on you, you can also get sick if you touch the things sick people touched, and then you touch your own face – your eyes, nose or throat.

Which brings us to a third recommendation: Limit physical contact. Avoid touching your face or shaking hands with others - and don't share utensils.