Summer heat, outdoor fun … and cold and flu symptoms?

The three may not go together in many people’s minds: partly owing to common myths about germs and partly because many viruses really do have lower activity levels in the summer.

But it is possible to get the sniffles — or worse — in the summer. Federal data released Friday, for example, shows COVID-19 is trending up in many parts of the country, with emergency department visits up among people of all ages.

The number of people seeking medical care for three key illnesses — COVID-19, flu and respiratory syncytial virus, or RSV — is low, according to data from the Centers for Disease Control and Prevention.

Flu is trending down and RSV was steady this week. But COVID-19 is trending up in many mid-Atlantic, southeast, Southern and West Coast states.

The expectation is that COVID-19 will eventually settle into a winter seasonal pattern like other coronaviruses, but the past few years have brought a late summer surge, said Dr. Dean Blumberg, chief of pediatric infectious diseases at University of California Davis Children’s Hospital.

Other viruses circulating this time of year include the one that causes “hand, foot and mouth” disease — which has symptoms similar to a cold, plus sores and rashes — and norovirus, sometimes called the stomach flu.

Many viruses circulate seasonally, picking up as the weather cools in the fall and winter. So it’s true that fewer people get stuffy noses and coughs in the summer — but cold weather itself does not cause colds.

It’s not just about seasonality. The other factor is our behavior, experts say. Nice weather means people are opening windows and gathering outside where it’s harder for germs to spread.

But respiratory viruses are around. When the weather gets hot and everyone heads inside for the air conditioning, doctors say they start seeing more sickness. In places where it gets really hot for a long time, summer can be cold season in its own right.

“I grew up on the East Coast and everybody gets sick in the winter,” said Dr. Frank LoVecchio, an emergency room doctor and Arizona State University researcher. “A lot of people get sick in the summer here. Why is that? Because you spend more time indoors.”

For people who are otherwise healthy, timing is key to getting any vaccine. You want to get it a few weeks before that big trip or wedding, if that’s the reason for getting boosted, doctors say. But, for most people, it may be worth waiting until the fall in anticipation of winter cases of COVID-19 ticking up.