Just in time for the holidays.

A rise in cases of a contagious lung infection across the country is causing alarm, especially for parents of young children.

Ali Rogin talks with an infectious disease specialist about what's behind the spike.

ALI ROGIN: The colder months usually mean more time indoors and higher rates of respiratory illnesses like the flu and COVID.

But now the CDC is tracking an uptick in cases of so called walking pneumonia.

The respiratory illness usually sickens school aged children and teens.

But this year, toddlers are being hit the hardest.

Dr. Donald Dumford is the infectious disease specialist at Cleveland Clinic.

Dr. Dumford, thank you so much for being here.

What is walking pneumonia and how does it differ from other types of pneumonia?

DR. DONALD DUMFORD, Infectious Disease Specialist, Cleveland Clinic: When we talk about walking pneumonia versus a more classic case of pneumonia, essentially it's typically causing a more mild course of illness.

If you think about a typical case of pneumonia, you're going to see abrupt onset of symptoms.

Whereas with walking pneumonia, which is typically caused by a bacteria called mycoplasma, you're going to see kind of a gradual onset over a few days leading to the illness.

So it's going to be a few days where you start to feel poorly, you start to get a cough, you start to get a fever, and really over about three to five days, you get a fever, headache, muscle aches and then start to get a pretty bad cough with it.

ALI ROGIN: What do we know at this point about why toddlers and other young children are being affected so much this year as compared to previous years?

DONALD DUMFORD: When you think about mycoplasma, typically we see that about every three to five years there's a cyclical increase in cases.

So normally, no matter what happens, every three to five years you see a rise.

We really didn't see that rise that we expected during the COVID pandemic.

So I think right now, because we have probably a higher number of susceptible people, you tend to that there is a lot of higher cases of severe illness, which is why we're seeing it a lot in the toddler population, along with the classical school age population, which is usually the population is most severely affected.

ALI ROGIN: How contagious is this?

DONALD DUMFORD: It is pretty contagious.

So, you know, the best way we look at this as far as the contagiousness is when you look at how it affects families and if it enters a family, you actually expect that about 80 percent of the other kids in the family are going to get sick, about 40 percent of the adults are going to get sick.

And probably, you know, all of us classically will get this probably three times in our lifetime.

One time during our childhood, one time when we're adults, whether our 20s, 30s, 40s, and then probably one time when we're elderly.

ALI ROGIN: Given that we're talking about a post-COVID spike here, are we seeing similar increases in other respiratory illnesses as well, or is it mainly confined to this one?

DONALD DUMFORD: Yes, definitely.

The other thing we've seen this year that has had a rather high spike in cases has been whooping cough, which is caused by a bacteria called Bordetella pertussis.

And it's actually something we, most of us or all of us get vaccinated for when we get vaccinated for tetanus and diphtheria.

Typically that similarly will have an increase in the number of cases about every four to five years.

But we've really seen a really high increase in the number of cases, probably about three or four times what we would otherwise expect, probably because of that having the lack of that increase in the number of cases.

And I guess the challenge to the population during the COVID years.

ALI ROGIN: And what should parents be looking for to spot whether this is walking pneumonia or something more similar to the common cold?

DONALD DUMFORD: Well, I think what you're going to see in your kids typically the common cold, you're really not going to have a fever with it.

A kid's going to have a runny nose, they're going to have a cough, but they're going to seem otherwise.

Well, your kid with mycoplasma, they're more likely to be complaining of feeling achy and they're going to seem a little more run down.

And typically it's going to be associated with a fever.

And when that fever sort of persists a little bit longer than you would otherwise expect with a virus, that's when you might start to worry about mycoplasma.

I think, really, whenever you see a child that's starting to act a little bit punky, starting to act a little bit run down, has a fever, it's time to call your pediatrician to check and see if they should get tested and if they would need antibiotics at that point.

We know that some of the measures applied during the height of COVID can help reduce illness, like washing your hands.

But do those measures still apply for other respiratory.

Respiratory illnesses, and are they things that people can employ to help mitigate these cases?

DONALD DUMFORD: Yes, definitely.

You know, really, as we enter respiratory season, you know, whether you're talking about COVID you're talking the flu, you're talking about this ongoing mycoplasma outbreak, you're talking about the other viruses that we tend to get.

You know, really all of those common sense measures are things that you should be employing.

Washing your hands is going to be imperative.

So you're not taking, you know, a virus you pick up on a table and take it up to your face, which is how you might get it off of a surface.

Making sure that you're covering your cough if you're sick is definitely a big thing.

It's really those common sense measures, they're going to protect your viewers from really all of these illnesses, whether it's mycoplasma or Covid or the flu or the other viruses we tend to see around this time of year.

ALI ROGIN: Dr. Donald Dumford, medical director for infection prevention at Cleveland Clinic, Akron, thank you so much for explaining all this to us.

DONALD DUMFORD: Thanks for having me, Ali.