Flu Symptoms

Medically Reviewed by Poonam Sachdev on August 29, 2024
8 min read

Influenza, aka the flu, is a highly contagious virus that infects your nose, throat, and sometimes your lungs. You can get sick by being around someone with the flu when they cough, sneeze, or talk. That's because the virus travels through the air on tiny droplets of fluid. If flu germs land on a surface and you touch them, then touch your nose, eyes, or mouth, you can get sick.

Some people who get the flu have only mild symptoms. Others become severely ill and need to go to the hospital. The flu can also be fatal. Getting a flu shot every year can help protect you.

Here are the specific signs and symptoms of flu to look out for.

Every year, about 20 to 40 million people in the U.S. get sick with the flu.

If you're one of them, you may think you have a cold at first. You could have a runny nose and a sore throat. But unlike a cold which gets slowly worse before it gets better, flu symptoms worsen very suddenly.

You'll likely start feeling sick 1 to 4 days after you come into contact with the virus.

If you have the flu, you could have some or all of these symptoms:

  • Fever
  • Chills
  • Runny or stuffy nose
  • Muscle and joint aches
  • Fatigue (feeling tired for no reason)
  • Shortness of breath
  • Warm, flushed skin
  • Red, watery eyes
  • Eye pain
  • Headache
  • Dry cough
  • Sore throat 

Vomiting and diarrhea aren't common flu symptoms in adults. They're more often seen in kids.

The symptoms of colds and flu can look a lot alike. Here’s how to tell which condition is making you sick:

Fever. It can last 3 or 4 days with the flu; it’s rare with a cold.

Aches. They might be severe with the flu; they’re mild with a cold.

Chills. You’re likely to have them with the flu, but they’re rare with a cold.

Fatigue. This is common with the flu and happens sometimes with a cold.

Sneezing. You’re more likely to have this with a cold than with the flu.

Cough and chest discomfort. This is common with both but tends to be severe with the flu.

Stuffy nose. This sometimes happens with the flu but is very common with a cold.

Sore throat. A scratchy, tender throat can happen with the flu but is more likely with a cold.

Headache. Your head is more likely to pound when you have the flu.

Generally speaking, cold symptoms are usually mild. Flu symptoms may become severe enough to lead to other health issues.

The flu and COVID are both upper respiratory infections, but they're not caused by the same type of germ. COVID is caused by a coronavirus known as SARS-CoV-2.

COVID spreads more easily than the flu and often causes more severe illness. You're also contagious for a longer period.

Without testing, it can be hard to tell whether you have COVID or the flu. They share many of the same symptoms. The exception is a loss of taste and smell for no reason (like having a stuffy nose) — which is common with COVID but rarely happens when you have the flu.

If you're sick with COVID, your symptoms can take longer to show up than the flu — about 2-14 days after you're exposed to the germ. Like a cold, COVID symptoms may also start out as mild, and then get worse over time.

If you think you have COVID and notice any of the following symptoms, it's important to go to the ER or call 911 right away:

  • Trouble breathing
  • Chest pain or pressure that doesn't go away
  • Pale, gray, or blue lips, nails, or skin
  • Finding it hard to wake up or remain awake
  • Confusion
 Flu Symptoms
Day 0Within 4 days of being infected, you're contagious. The virus is still building up in your body so you won't show any symptoms yet.
Day 1-2All of a sudden, you feel awful. Fever, headache, chills, and muscle aches tend to begin before a sore throat. Expect to be exhausted.
Day 3-4Day 3 may be when your symptoms are at their worst. Even so, you're already less contagious.
Day 5-6Your fever and headache should improve. But you'll likely still feel tired and may not have much of an appetite.
Day 7You should be on your way to recovery, even if some of your symptoms linger. You'll also no longer spread the flu to others.

If you do get sick, you'll need to take care of yourself. In most cases, the best things to do when you have the flu are:

Get lots of rest. When you're sick, your body needs more sleep to regulate your immune system.

Drink plenty of fluids. Sweating from a fever or chills, diarrhea, and vomiting can all put you at risk of getting dehydrated. While water is the best choice for staying hydrated, fluids such as tea, electrolyte drinks, broths, and soups are also good options. Warm fluids can help soothe your throat. Try to avoid sugary sodas and fruit juices as well as caffeinated drinks.

Apply heat on achy muscles. A heating pad or warm (not hot) shower or bath can provide some relief. But only try this if you aren't running a fever.

Turn on a humidifier. Adding more moisture in the air can ease a stuffy nose and help you cough less.

Avoid contact with other people. You don't want to spread your germs around. Stay home from work or school until you recover.

Try over-the-counter (OTC) pain relief. No one under the age of 18 should take aspirin. It raises your risk for a rare brain disease called Reye's syndrome. If you're not sure whether to take acetaminophen or a nonsteroidal anti-inflammatory drug (NSAID) such as ibuprofen, ask your doctor. Based on your symptoms, they can also suggest an OTC cough suppressant (which can reduce coughing) or expectorant (which helps get rid of mucus in your throat and lungs).

Flu medications

Antibiotics can't treat the flu. But if you're at risk of complications from the flu, your doctor might prescribe antiviral medication.

These types of drugs work best when you take them no more than 48 hours after your symptoms start. Antivirals can decrease how severe your symptoms are and reduce the time that you're sick. They may also help prevent serious health issues related to the flu.

Antiviral medications include:

  • Baloxavir marboxil (Xofluza), which is given as a pill or liquid. It's not safe for everyone. For instance, you can't take it if you're pregnant or nursing.
  • Oseltamivir (Tamiflu), which your doctor can prescribe as either a pill or liquid. For it to work, you'll need to take it for several days in a row.
  • Peramivir (Rapivab) is given directly in your vein (in an IV). You only need one dose.
  • Zanamivir (Relenza) is breathed in through an inhaler. It's not a good choice if you have breathing issues, such as asthma.

Antivirals aren't supposed to replace a flu shot. Your best option is to try not to get the flu in the first place. The CDC recommends a yearly flu vaccine (as either a shot or a nose spray) for everyone 6 months or older. Studies show that past vaccines have been able to decrease the risk of flu by 40%-60%.

For most people, the flu is a short-term illness that runs its course in 5 to 7 days. If you've had a flu shot, you may start feeling better even sooner. But everyone is different. It may take you longer to feel back to your usual self. And some symptoms, such as fatigue, may persist longer than others.

If you're in pretty good health, you probably won't have serious problems from the flu. But many different things can raise you at risk for complications, including:

  • Being younger than 5 years old (especially between the ages of 6 months and 2 years old)
  • Being older than 65
  • Being 19or younger and taking long-term aspirin therapy
  • A weakened immune system (for instance, because you're going through chemotherapy or live with a condition such as AIDS)
  • Pregnancy
  • Living in a long-term care facility
  • Asthma
  • Diabetes
  • Heart disease
  • Liver or kidney issues
  • Metabolic disorders
  • Nervous system disorders
  • A body mass index (BMI) of 40 or higher

If you're Black, Hispanic, American Indian, or Alaska Native, you're also at higher risk of flu complications.

Flu complications can include:

If your symptoms are mild and you're not at risk for complications, you probably won't need to see your doctor. Taking care of yourself at home will probably be enough to recover from this illness.

But if you come down with the flu and are concerned, let your doctor know right away. They may prescribe antiviral medicine, give you specific advice to follow, and keep a closer eye on your health.

When to seek emergency care for flu symptoms

Whether or not you fall into a "high-risk" group, get medical help right away if you have any of the following:

  • Trouble breathing
  • Shortness of breath
  • Pain or pressure in your chest or belly
  • Sudden dizziness
  • Confusion
  • Severe vomiting
  • Not peeing as much as you usually do
  • Intense muscle cramps
  • Fever that goes away and comes back
  • Cough that improves and comes back

The signs that your child needs emergency care include:

  • Fever above 104 F (in babies younger than 12 weeks, any fever is an emergency)
  • Fast or labored breathing
  • Ribs that pull in each time they breathe
  • Skin or lips that look blue
  • Intense muscle cramps 
  • Not peeing for 8 hours
  • No tears when crying
  • Lethargy (not moving around or wanting to play)
  • Fever that keeps coming back
  • Cough that won't go away, or goes away and comes back
  • Seizures

If you or your child have an ongoing medical condition, it's also a good idea to check in with your doctor. There may be other symptoms that signal you need to go to the ER. 

Seasonal flu follows a pattern, starting in the fall and ending in the spring. The first sign is often a sudden rise in the number of school-age kids getting sick. This is usually followed soon after by illness in other groups, such as parents and caregivers.

The CDC advises getting a flu shot in October, around the beginning of flu season. That's because it will take 2 weeks to get into your system and fully protect you. The flu virus tends to peak in December and January. But you can be infected anytime, so it's never "too late" for a flu shot.

Self-care is usually enough to treat flu symptoms, but always call your doctor with concerns. The complications from flu can be severe and sometimes fatal. A flu shot can help protect your health by reducing how severe your symptoms are and how long they last.