Cold and Flu Symptoms

How do you tell the difference between the flu and a common cold? Both are contagious viral respiratory infections, and they share many symptoms. But there are key differences in how a cold and the flu set in and progress.

Flu symptoms often start with a fever that comes on quickly, whereas cold symptoms typically begin slowly and are more likely to include a runny or stuffy nose. Flu symptoms tend to be more severe than those of a cold, with viral infection of the throat, nose, and lungs more likely to lead to a range of potentially serious complications, according to the Centers for Disease Control and Prevention (CDC).

Learn More About the Differences Between Cold and Flu Symptoms

Can a Cold Turn Into the Flu?

The common cold and the flu are caused by different viruses. The flu is caused by influenza viruses, while the common cold can be caused by 200 other distinct viruses. Most colds are caused by a type of virus called a rhinovirus, per the CDC.

Every year, adults in the United States have between two and three colds, on average, while children have more, according to the CDC.

Cold symptoms typically appear about one to three days after exposure to a cold-causing virus, with most cases peaking within two to three days.

Early signs of a cold may include fatigue, sneezing, and a sore throat. During the first day or two, you may feel as if you have an itch or something stuck in your throat. Then you may begin to sneeze more than normal. When nasal symptoms start, the nose produces clear mucus, but the discharge may later change to white, yellow, or green, notes the CDC.

Per the CDC, symptoms of a cold can include:

• Runny or stuffy nose
• Sneezing
• Postnasal drip
• Coughing
• Sore throat
• Watery eyes
• Fever (most people with a cold will not have a fever)

The full life cycle of a cold is usually between 4 and 10 days, according to the Merck Manual, although some people may have a cough that lasts into the second week. A cold may last longer or be more severe in people who have underlying health issues, says the CDC.

The contagious period for a cold can vary, but is usually at its peak during the first day or two after symptoms develop, notes the Merck Manual.

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Unlike cold symptoms, which develop gradually, flu symptoms usually come on suddenly. Cold and flu symptoms may overlap, but the latter tend to be more severe. There is also a greater risk of complications with influenza, such as pneumonia, according to the CDC.

“[With the flu] you’re sick for a longer period of time, and recovery will take a long time. Getting your energy back can take a couple of weeks, even in a healthy individual,” says Catherine Troisi, PhD, an associate professor in the management, policy, and community health division and the epidemiology division at University of Texas School of Public Health in Houston.

According to the CDC, symptoms of the flu typically include:

• Fever (but not everyone with the flu develops a fever)
• Runny or stuffy nose
• Coughing
• Sore throat
• Fatigue
• Muscle or body aches
• Headaches
• Vomiting and diarrhea (more common in children)

The CDC lists a number of emergency warning signs that should prompt immediate medical attention, including:

• Difficulty breathing or shortness of breath
• Persistent pain or pressure in the chest or abdomen
• Persistent confusion, dizziness, unconsciousness
• Seizures
• Not urinating
• Severe muscle pain
• Severe weakness or unsteadiness
• Fever or cough that improves, then returns or worsens
• Worsening of chronic medical conditions

The flu usually lasts for several days or occasionally weeks, with the most severe symptoms improving after two or three days. But weakness, fatigue, and dry cough caused by influenza can persist for six to eight weeks, according to the Merck Manual.

Occasionally people who are infected with the flu virus have no fever, at least not at the onset of other symptoms. Some people go on to develop a fever later, and some do not. “Children tolerate high fevers better than adults, but 102 degrees F is a significant fever, especially for adults,” says Randy Bergen, MD, a pediatric infectious disease consultant in Walnut Creek, California.

With or without a fever, influenza is highly contagious. While people with the flu are most contagious in the first three to four days after the illness begins, they can infect others as early as one day before their symptoms set in and up to a week after becoming sick. Some people, including very young children and people who are immunocompromised, may be contagious for even longer, notes the CDC.

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Flu symptoms in children are similar to those in adults, but children are more likely to experience vomiting and diarrhea, according to the Merck Manual.

Babies who have the flu seem sicker, fussier, and more uncomfortable and unhappier than babies who have a cold, according to the March of Dimes. Babies and children under age 5 are especially at risk for serious flu-related complications. Children with chronic health problems, such as asthma, have the highest risk of complications, the March of Dimes notes.

Per the CDC, emergency warning signs that should prompt immediate medical attention include:

• Fast breathing or trouble breathing
• Bluish lips or face
• Ribs pulling in with each breath
• Chest pain
• Severe muscle pain (child refuses to walk)
• Dehydration (no urine for eight hours, dry mouth, no tears when crying)
• Not alert or interactive when awake
• Seizures
• Fever over 104 degrees F
• Any fever in children younger than 12 weeks
• Fever or cough that improves, then returns or worsens
• Worsening of chronic medical conditions

Healthcare providers generally diagnose the flu on the basis of symptoms and their clinical judgment, according to the CDC, which says most doctors opt not to use a flu test because the results generally won’t affect treatment.

When doctors decide to use a test, they have a number of types to choose from, ranging in speed and accuracy.

Should I Call in Sick for a Cold or Only the Flu?

The CDC advises workers with flu symptoms to stay home until at least 24 hours after their fever (temperature of at least 100 degrees F) is gone or at least four to five days after the onset of symptoms.

The CDC also recommends that people who have a cold stay home to prevent spreading it to other people and encourages parents to keep children with colds home from school or daycare.

What Should I Eat When I Have the Flu to Alleviate Symptoms?

For people with the flu, experts tend to focus on foods and beverages that can prevent dehydration. The Mayo Clinic recommends drinking plenty of water and juice, and consuming warm soups.

RELATED: 10 Simple and Natural Ways to Boost Your Immune System

How to Prevent a Cold or the Flu

The CDC outlines a number of steps to stave off a cold:

• Wash your hands often with soap and water. If soap and water aren’t available, use an alcohol-based hand sanitizer.
• Avoid touching your face with unwashed hands
• Keep your distance from people who are sick

While the advice above also applies to the flu, the best prevention is vaccination, says the CDC, which recommends that every person age 6 months and older get an annual flu vaccine by the end of October.

If you have the flu and need to leave your home or be out in public, a mask can reduce the spread of infection, notes the Mayo Clinic.

RELATED: No Flu Shot and Now You Have the Flu: What to Do?

Additional reporting by Pamela Kaufman.