Three different viruses are sending children and adults to hospital emergency rooms across the United States this winter. Not only is the coronavirus making a comeback because of new immune-evasive variants, but influenza and the respiratory syncytial virus are also spreading earlier than usual and at record rates.

Most people were able to avoid the flu and other infections like R.S.V. for the past few years while the coronavirus was new and a large portion of the population was taking steps to slow the spread of Covid, which also slowed the spread of other viruses. Our immunity to these viruses may have waned in that duration, and many children born during the pandemic never gained any immunity at all. As a result, more people are vulnerable to common winter viruses now that much of the country has set aside masks and social distance and is traveling and gathering indoors more.

To make matters worse, we may also be more susceptible to severe illness if we do fall ill, said Dr. Priya Soni, a pediatric infectious disease specialist at Cedars-Sinai Guerin Children’s in Los Angeles.

“You can get really sick, perhaps more so than you would have normally because your immune system really hasn’t been challenged as much recently,” she said.

Here’s a guide to spotting symptoms — and what to do to confirm and treat the various infections.

Mark your symptoms below and then sort by selecting an illness.

The flu, Covid and R.S.V. have symptoms that overlap — and also can look quite similar to those of the common cold, which is caused by a variety of viruses. You can develop a cough, fever, headache or runny or stuffy nose with any respiratory virus, said Dr. Michael Chang, an infectious disease specialist at Memorial Hermann Health System in Houston. But one way to differentiate among viral infections is to monitor how quickly your symptoms ramp up.

Symptoms of the flu come on quickly — often just one day after exposure to someone who was sick — and can be felt all over the body. People with the flu often describe feeling like they’ve been hit by a truck, Dr. Chang said. With colds, symptoms may take two or three days to appear and are much milder. Covid and R.S.V. have even longer incubation periods. It can take an average of five days from exposure to the coronavirus to the development of early Covid-19 symptoms, though newer variants may result in an active infection as soon as three days after exposure. R.S.V. takes about four to six days. With Covid and R.S.V., symptoms also build slowly: You may start out feeling sniffly, and then develop a cough or a headache the next day and a fever the next.

Adults afflicted with the flu are likely to experience very high fevers, up to 103 or 104 degrees, Dr. Chang said. But fevers rarely accompany colds, particularly in adults. And those with Covid-19 and R.S.V. experience mild fevers or none at all. “Especially with the newer variants and people having been exposed through immunization or if they’ve had an infection before, we’re seeing that more patients are now only mildly symptomatic, and they only have low-grade fevers, around 99 or 100,” Dr. Chang said.

Other, less common symptoms can also help you differentiate between illnesses. For example, the flu and Covid-19 can cause diarrhea and vomiting; these gastrointestinal issues are more likely in young children. Covid-19 can also result in a sudden and severe loss of taste or smell, which is very different from the temporary dampening of these senses when you have a stuffy nose with a cold or flu, Dr. Chang said.

People with R.S.V. are less likely to feel the full-body fatigue and muscle aches that come with the flu or Covid-19. Instead, R.S.V. is often accompanied by a very wet and forceful cough. “Of the three viruses, R.S.V. will tend to have the most mucus in your nose and throat and the most congestion,” Dr. Chang said.

Another distinctive symptom of R.S.V. is wheezing, particularly in young children. You can hear it as a high-pitched whistling or crackling sound when a sick child exhales. And it may be accompanied by faster or more labored breathing overall.

Looking at community transmission rates in your area is another way to narrow the possibilities for which virus you might have. While official Covid case numbers may be undercounted because of increased home testing, you can still check the Centers for Disease Control and Prevention’s Covid-19 map, in addition to relying on local news and social networks for a sense of whether members of your community are sick.

Flu and R.S.V. tests have to be conducted in doctor’s offices or clinical laboratories and may be more accurate representations of case rates across the country. The C.D.C. publishes this data in weekly flu maps and R.S.V. trends.

In general, adults over 65 are at highest risk for all three viruses. That’s because the body’s ability to fight off viruses naturally decreases with age. And older adults are more likely to have medical conditions that can put them at risk for severe symptoms and complications if they get sick.

Young children also tend to experience more severe symptoms of the flu and R.S.V. because their immune systems don’t yet know how to fight off pathogens properly, Dr. Soni said. Infants and toddlers also have smaller airways that tend to become congested easily, making their illness more likely to progress from mild to severe and cause difficulty breathing.

Children are more likely to experience asymptomatic or mild Covid-19 infections compared with adults, Dr. Chang said, though “it’s not really well understood why that is.”

The only way to definitively diagnose or rule out one of these viruses is to get tested. While you can easily check for Covid-19 at home, there are no rapid home tests for the flu or R.S.V. There is a combined P.C.R. test developed by Labcorp that allows you to take a nasal swab for Covid-19, flu and R.S.V. and mail the sample to a lab. But results take one or two days.

Most clinics, urgent care centers and hospitals also have combination tests for all three viruses, Dr. Chang said. But some doctors may forgo a diagnostic P.C.R. if you have a mild infection. That’s because the symptom management for most people will stay the same regardless of which respiratory virus they have, Dr. Soni said. (Those who fall in high-risk categories or are sick enough to be hospitalized may receive antiviral treatment for their infection, such as Paxlovid for Covid-19, Tamiflu for the flu and ribavirin for R.S.V.)

“The best things most people can do are stay hydrated with lots and lots of fluids, keep a humidifier running in the room and help younger kids with nasal suctioning devices,” Dr. Soni said. Children and adults who are generally healthy and do not have other underlying medical conditions, she added, can try over-the-counter medicines for symptom relief and will typically recover with a little bit of time.