What Does Incubation Period Mean?

Medically reviewed by Renee Nilan, MD

An incubation period is the time from exposure to an infectious agent to when you actually develop symptoms. Most people do not develop symptoms immediately after exposure to a virus or bacteria. It will take time for the virus or bacteria to infect you enough to feel sick.

For example, the Centers for Disease Control and Prevention (CDC) report that COVID-19 symptoms may present anywhere from two to 14 days after exposure to the virus. A food-borne bacterial infection like Salmonella can cause symptoms within 6 hours to six days. Conversely, the flu virus has an incubation period of one to four days.

This article will cover incubation period examples, factors that might affect the incubation period, when to quarantine, what happens after the incubation period, and more.

Incubation Period: A Simple Definition with Examples

The incubation period is the time from acquiring an infection until the onset of symptoms.

You will not develop symptoms immediately after acquiring a virus, bacteria, or fungus. It will take time for the organism to multiply and infect enough of the body's cells for your body to react to it and for you to feel sick.

You may not know you have an infection during the incubation period, but you will feel sick once the infection has developed enough.

Incubation periods in infectious diseases can help provide information during an outbreak, including when symptoms start and when someone is most likely to transmit the infection. They can also offer insight into disease severity and how long an illness might last. Incubation periods will vary based on the type of virus or bacteria.

COVID-19

The incubation period for COVID-19 will vary by the virus variant. Meta-analyses of studies published in 2021 identified an average incubation period of 6.5 days from exposure to onset of symptoms. Studies on the Delta variant have reported an incubation period of 4.3 days, and studies on Omicron have reported an average incubation period of three to four days.

COVID-19 testing can detect SARS-COV-2—the virus that causes COVID-19—at specific points during the infection, often when symptoms start. If you test too early or too late, you may get a false negative and unknowingly spread the virus.

Start of an incubation period: The average COVID-19 incubation period is around five days. Symptoms of COVID-19 can appear as early as two days and as late as 14 days after exposure to the virus. The incubation period starts when you first come in contact with the virus and become infected.

During this time, the coronavirus will invade cells in your body and replicate. You might be contagious during this time, and studies have found that you are more likely to spread the virus during incubation.

This is often because people who do not know they have the virus will not take isolation measures to prevent infection spread. But anyone with COVID-19 can spread the virus despite symptoms, variant type, and vaccine status.

End of incubation period: The start of symptoms ends the incubation period. Symptoms and symptom severity will vary from person to person. Once you experience symptoms, you can quickly spread the virus via tiny respiratory droplets or aerosol particles that leave the body with sneezing, coughing, or speaking.

Read Next: COVID-19 Incubation Period: Average Number of Days After Exposure

Influenza

Influenza, referred to as "the flu," is an infectious respiratory virus caused by influenza viruses. Symptoms can range from mild to severe and may include fever, sore throat, runny nose, muscle pain, and headache.

Start of incubation period: The incubation period for the flu is between one and four days, but this varies from person to person. The average incubation period is two days, which means most people develop symptoms two days after exposure to an infected person.

The incubation period may be affected by infectious dose (amount of virus you are exposed to), the route infection (how the virus enters the body), and your pre-existing immunity (if you had previously been exposed or vaccination status).

End of incubation period: Symptoms of the flu start at the end of the incubation period. But you can spread the virus to others at least a day before you notice symptoms. You are most contagious the first day you experience symptoms. Older adults, children, and people with weakened immune systems might be contagious for longer.

Related: How Long Is the Flu Contagious?

The Common Cold

The common cold is a viral infection of the upper respiratory tract, affecting the nose, throat, sinuses, and larynx (voice box). Symptoms include sore throat, runny nose, sneezing, coughing, headache, and low-grade fever—between 99 degrees F (37.2 C) and 100.4 degrees F (38.0 C). Different viruses are associated with the cold, including rhinoviruses, adenoviruses, and enteroviruses.

Start of incubation period: The incubation period for the common cold is short, ranging from two to five days. The appearance of symptoms will depend on the virus you were exposed to.

For example, if you have been exposed to a rhinovirus, you can expect symptoms between 12 to 72 hours after exposure. Other cold viruses may have more extended incubation periods.

End of incubation period: The incubation period for the cold will end when symptoms start. You may experience a scratchy throat, a runny nose, or fatigue early on, and after that, sneezing and a sore throat. As soon as symptoms appear, you are contagious and can spread the virus.

Symptoms will peak by day four, and you will experience body aches, coughing, and congestion as your body fights off the virus. Most people recover within a week, but full recovery could take up to two weeks.

Related: What Is a Cold?

Chickenpox

Chickenpox is an illness caused by the varicella-zoster virus. It causes an itchy rash and small fluid-filled blisters. It can spread quickly to people who have not had it before and who are unvaccinated.

Start of incubation period: The incubation period for the varicella-zoster virus is 10 to 21 days after exposure. During the incubation period, the virus will enter the body and spread, but there will be no symptoms.

End of incubation period: At the end of the incubation period, a prodromal stage of one to two days starts where you will experience flu-like symptoms, including fever, sore throat, headache, and fatigue. Adults are more likely to have these symptoms, while children will only develop a rash.

You are most contagious during this time and can spread the virus to others. Once the rash appears, it will start as small red bumps on the face, chest, and back, spreading to other skin areas. These bumps will become fluid-filled and eventually crust over and fall off.

As blisters heal, they will be itchy and painful. Full recovery could take up to two weeks.

Related: Incubation Periods of Childhood Diseases

Bacterial Infections

Bacterial infections can affect various body areas, including the skin, lungs, brain, and blood. You can get a bacterial infection after direct contact with bacteria from an infected person (i.e., touching, kissing, coughing, sneezing, etc.), exchanging body fluids, contact with infected surfaces, and consuming contaminated food or water.

Examples of bacterial infections include food poisoning, respiratory infections, skin infections, gastrointestinal infections, urinary tract infections, and sexually transmitted infections (STIs). Incubation periods and symptoms will vary based on the type of bacteria involved.

Strep throat, a respiratory infection caused by group A streptococcus, has an incubation period of two to five days. You then develop a sore throat, fever, and other symptoms.

E. coli infections (from contaminated food and water) can have an average incubation period of three to four days, while Bacillus cereus infections (from food improperly stored) can have an incubation period of 30 minutes to 15 hours. The incubation period for bacterial gastroenteritis caused by Campylobacter bacteria is one to five days.

Each STI has its own incubation period. For example, the incubation period is one to 14 days for gonorrhea and seven to 21 days for chlamydia.

Related: The Incubation Period of Common STIs

Antibiotics are used to treat bacterial infections. You are no longer contagious after treatment, although this can sometimes differ depending on the bacterial cause.

Read Next: Viral vs. Bacterial Infection: What’s the Difference?

Factors That Change the Incubation Period

Incubation periods will vary depending on the infection type, the pathogen's strength, and exposure dose. Vaccine status and prior infection usually do not affect incubation periods.

Infection Type

Depending on the cause of the disease, the incubation period could be a few hours to several months. For example, the incubation period for the stomach flu could be less than 24 hours, while the incubation period for hepatitis A could be up to 50 days.

The incubation period will start after exposure to the infection source and before symptoms start. Exposure to the infection source does not mean you will become infected or that you will develop symptoms. One 2021 research review found that asymptomatic (no symptoms) COVID-19 affects about 40.5% of people who test positive for the virus.

Strength of the Pathogen

Viral and bacterial pathogens can cause minor harm or severe harm. The incubation period may depend on the pathogen's strength based on the harm it might cause.

For example, highly virulent (harmful) pathogens will almost always lead to disease when they hit the body and have a short incubation period. Less virulent pathogens or avirulent (not harmful) ones may cause an initial infection that takes longer to cause symptoms. These types of pathogens will cause a mild disease or may be asymptomatic.

Exposure Dose

The exposure dose (also called viral dose) is the amount of infectious virus transmitted from one person to another. Some studies on COVID-19 find that higher or more potent viral doses can make some people more contagious than others.

The likelihood of becoming infected with COVID-19 may increase based on how much exposure you have had to an infected person.

Quarantine During Incubation Period

To quarantine means staying away from others to prevent disease transmission. This time varies based on the source of infection.

Guidance on isolating from others with COVID-19 comes from the CDC. They recommend isolating from others if you have COVID-19 or are sick and suspect you have COVID-19 but do not yet have test results.

If you test positive for COVID-19, you should isolate for at least five days. This is the time when you are most contagious.

You can end your isolation after five days if you are fever-free for at least 24 hours without fever-reducing medications. If you have a more severe illness, you should isolate for at least 10 days.

If you test positive and do not have symptoms, you should still isolate for five days, and if you continue to have no symptoms, you can end your isolation, says the CDC. You can still spread the virus without symptoms, so you should isolate.

Related: Do Vaccinated People Have to Quarantine If They're Exposed to COVID?

After Incubation Period

The incubation period ends when you first notice symptoms of an illness after exposure to a pathogen.

Are You Still Contagious?

For most viral illnesses, you will be contagious at some time during the incubation period and then throughout the time symptoms are present. Some viruses can still be transmitted after you no longer have symptoms.

People with COVID-19 are the most contagious in the first two days before and three days after symptoms develop. You could be contagious up to 10 days after symptoms start, especially if you have a more severe disease.

For the common cold and the flu, you can transmit the virus to others at least a day before you notice symptoms, and you are most contagious the first day you experience symptoms. You will want to isolate for at least a few days after symptoms start and until you are fever-free for at least 24 hours.

Chickenpox is highly contagious, and you will be the most contagious one to two days before you develop a rash. You will continue to be contagious until blisters have crusted, so you must isolate for the entire time to avoid spreading the virus to others. According to the CDC, you are no longer contagious after you have had no new lesions for 24 hours.

Some bacteria and viruses may remain in the body after infection and can continue to be transmitted to others after symptoms resolve. Examples include herpes, hepatitis, methicillin-resistant Staphylococcus aureus (MRSA), and Salmonella.

Some ways to reduce the risk of infections are handwashing, not sharing personal items, covering your mouth with sneezing and coughing, getting vaccinated, wearing face masks, practicing food safety, and practicing safe sex (i.e., limiting partners and using condoms consistently).

Summary

The incubation period of a virus or bacteria is the time it takes to develop symptoms after infection. The COVID-19 incubation period is two to 14 days. Timelines for incubation periods will vary based on the infection type, strength of the pathogen, and exposure dose. Vaccine status and prior infection usually do not affect incubation periods.

You are typically contagious during the incubation period. The incubation period ends when symptoms start, but you are still contagious and must isolate for at least a few days. You might be the most contagious the first day or two after symptoms start.

You should isolate from others if you are sick to prevent spreading infection. For COVID-19, you will also want to isolate if you suspect you have COVID-19 and if you test positive but do not have symptoms. You can still spread the virus even if you are asymptomatic.

Read the original article on Verywell Health.