Contagiousness refers to the ability of an infected individual to transmit a disease to others. The timing of when a person is most likely to spread an illness, relative to when they show symptoms, is not straightforward and varies greatly depending on the specific pathogen. Understanding these distinct phases of transmission is important for managing public health and preventing the spread of infections.
Spread Before Symptoms Appear
The period before symptoms become noticeable is called the incubation period, during which an individual can still be highly contagious. For example, with the common cold, individuals can spread the virus for a few days before any symptoms appear, and this period can extend up to two weeks in total. Similarly, people infected with influenza can transmit the virus approximately one day before symptoms emerge. This occurs because the pathogen, whether a virus or bacteria, is actively replicating within the body, reaching sufficient levels to be shed and potentially infect others, even without triggering a noticeable immune response that causes symptoms.
With COVID-19, individuals are often most infectious one to two days before symptoms begin. This pre-symptomatic spread poses a unique challenge, as people can unknowingly transmit the illness before they even realize they are sick.
Contagiousness During Active Illness
Once symptoms appear, contagiousness often continues, and for many illnesses, the peak period of transmission can occur early in this symptomatic phase. For influenza, individuals are most contagious during the first three to four days of symptoms. Similarly, with the common cold, the period of highest infectiousness is generally within the first two to three days when symptoms are most severe. These symptoms, such as coughing and sneezing, directly facilitate the expulsion of respiratory droplets containing the pathogen, making transmission more likely. For COVID-19, contagiousness can peak around day four of symptoms, though individuals can remain infectious for an average of eight days.
Transmission After Symptoms Subside
After symptoms have improved or completely disappeared, the duration of contagiousness varies significantly depending on the illness. For many common respiratory viruses like the flu, contagiousness generally decreases significantly once symptoms resolve, although individuals might still be able to spread the virus for up to seven days after becoming sick, especially in young children or those with weakened immune systems. With the common cold, while most people recover in three to ten days, some can remain contagious for up to two weeks, even after symptoms lessen.
For COVID-19, most individuals with mild to moderate illness are likely not contagious after ten days from symptom onset, especially if symptoms have fully resolved. However, those with severe illness or compromised immune systems might remain infectious for up to three weeks or even longer. Some viruses, like norovirus, can continue to be shed in stool for two weeks or more after a person feels better, making proper hygiene important even post-recovery.
Reducing the Risk of Transmission
Regardless of the phase of illness, several actions can help reduce the risk of spreading infections:
- Frequent handwashing with soap and water for at least 20 seconds, or using an alcohol-based hand sanitizer, is an effective way to remove germs. This is particularly important after coughing, sneezing, or being in public places.
- Avoiding touching your face, especially your eyes, nose, and mouth, with unwashed hands can also prevent self-infection and transmission.
- When coughing or sneezing, cover your mouth and nose with a tissue or your bent elbow to contain respiratory droplets. Used tissues should be disposed of immediately in a closed bin.
- Staying home from work, school, or social gatherings when feeling unwell is a straightforward way to prevent spreading pathogens to others, particularly during periods of high contagiousness.
- Maintaining physical distance from others, especially in crowded or poorly ventilated indoor spaces, and wearing a well-fitting mask when distancing is not possible can further reduce transmission risks.