The common cold and the flu have many features in common: Both of them are caused by viral infections, show similar symptoms, and usually can be treated at home. They also typically develop in stages, with certain symptoms emerging as the infection develops. But there are differences in the onset, severity, and duration of the typical cold or bout of influenza.
What Are the Stages of a Cold?
Cold symptoms can vary from person to person, but they generally appear about one to three days after exposure to a cold-causing virus. In most cases, cold symptoms will maximize around day four and taper off around the seventh day.
The signs that you have a cold usually develop slowly. The most common cold symptoms include fatigue, sore or scratchy throat, nasal congestion or stuffiness, and a runny nose, accompanied by sneezing and coughing. Fever is not typical with a cold, but a low-grade fever isn’t out of the picture.
The mucus discharged by a runny nose may change color over the course of the illness, starting out clear and becoming thicker, yellow, or green. Postnasal drip, in which mucus accumulates or drips in the back of the throat, can further worsen a sore throat or cough.
The full life cycle of a cold is usually between a week and 10 days. A cold may last longer or be more severe in people who have immune problems or other underlying health issues.
If your symptoms lasts more than two weeks or keep reoccurring, then something else may be going on, such as allergies, sinusitis, or a secondary infection.
“Fever is an important sign,” says Norman Edelman, MD, a professor of preventive medicine, internal medicine, and physiology and biophysics at the State University of New York at Stony Brook in Long Island. Adults with a fever of 102 F or higher and children with a fever of 103 F or higher should see a doctor.
The contagious time frame for the common cold has its own life span, usually starting a couple days before cold symptoms kick in and continuing for a few days afterward.
What Are the Stages of the Flu?
Flu symptoms usually start within one to four days after infection. Unlike a common cold, the effects of an influenza virus infection can come on very suddenly.
The first signs of the flu are often a fever or chills, accompanied by headache, sore throat, dry cough, runny nose, muscle aches, and fatigue.
As the illness progresses, a person may have warm, flushed skin, watery or bloodshot eyes, a severe cough that produces phlegm, and nasal congestion. Nausea and vomiting may also occur, especially among children.
A bout of the flu typically lasts one to two weeks, with severe symptoms subsiding in two to three days. However, weakness, fatigue, dry cough, and a reduced ability to exercise can linger for three to seven days.
How Long Is the Flu Contagious?
A 2013 survey carried out by the National Foundation for Infectious Diseases revealed that 41 percent of people think the flu is only contagious after symptoms start. (1) That’s not true.
An adult infected with influenza may be contagious from day one before symptoms start until five to seven days after becoming sick. Children may continue to be contagious for longer than seven days.
Staying home until your contagious timeline has likely passed will help you avoid passing germs on to other people.
Other simple steps can keep you from spreading infection to others or picking up a virus from other people around you at school, work, or at home.
“It’s really basic public health practices,” says Catherine Troisi, PhD, an associate professor in the divisions of management, policy, and community health and epidemiology at the University of Texas School of Public Health in Houston. “You should wash your hands, stay home if you’re sick, get enough sleep, and eat well.”
What If a Cold or the Flu Refused to Go Away?
When complications develop, a person will likely be sick for longer than a week or two, depending on the severity of the complication, how quickly a person receives treatment for it, and how well the patient responds to treatment.
Signs of severe complications that should prompt you to seek medical attention include the following:
- Difficulty breathing or shortness of breath
- Purple or blue discoloration of the lips
- Pain or pressure in the chest or abdomen
- High fever
- Sudden dizziness
- Severe or persistent vomiting
- Flu-like symptoms that improve but then return with fever and worse cough
Even in healthy people who don’t develop complications, the flu can cause symptoms that persist for weeks, including:
- Low appetite
- Dry cough
- Airway irritation that affects how long you can be active
- Loss of sense of smell, which in rare cases becomes permanent
How Long Does Immunity Last?
With most viral illnesses, once you have been infected with it or have been vaccinated against it, you’re immune for life.
But with the flu, however, immunity doesn’t last that long.
A study published in March 2017 in the journal Clinical Infectious Diseases confirmed that immunity declines over the months following vaccination or infection. (2)
Getting vaccinated yearly is important to lower your likelihood of getting the flu. It takes about two weeks to develop immunity to the flu, and experts recommend getting vaccinated before flu season is in full swing to ensure adequate protection. Even getting a flu shot as late as January can help protect against the flu, particularly if the season peaks in February or March.
“I think part of the problem with getting people vaccinated is people don’t understand how serious [the flu] can be,” says Dr. Troisi. “They confuse it with the common cold. But if you actually have the flu, you can get very sick.”