by Joshua Horn

Created on: February 21, 2011

Influenza is a deadly virus that kills many worldwide every year. But for those with strong immune systems, it is usually just a frustrating nuisance that can sideline productivity for nearly a week. While we are all busy in the fast-paced world, it is important to recognize and understand the contagious period of the flu, so as not to infect our friends, family or coworkers.

From just a few days to several weeks, many estimates are often provided for the length of the "contagious period" for the flu. While these estimates are all correct, they are only correct in certain circumstances. A good look at the underlying mechanism of influenza infection can lead to a better understanding of how long people are infectious when carrying the virus. As a rule of thumb, it is generally accepted that most people will be contagious for about 8 days after the symptoms appear, but this may be much longer depending on each person's unique immune response.

The influenza virus, in many ways, is a classical virus with replication and incubation periods similar to many other viruses. The infected individual catches the virus through some means of transmission, usually bodily fluids such as saliva or mucus. After initial infection, the virus "incubates" by commandeering the replication systems of healthy to build more virus particles. When the cells have swelled with newly synthesized virus particles, they burst and the viral particles spread to other cells and tissues.

You would assume that while the virus is incubating, it is unlikely that the virus can be easily spread. This is a solid assumption that generally holds true. Upon immediate infection, the host cannot transmit the virus, but their infectiousness increases quickly the longer the virus incubates.

Once the virus has spread throughout the body and the symptoms of influenza have appeared, the host is more contagious than ever. This holds true for two reasons:

First, the virus particles are most abundant at this point. As the body builds immunity, the particles are destroyed and the human becomes healthy again. This means that a person's contagious period begins to decline, though it doesn't immediately disappear, as soon as the immune system mounts its response.

Secondly, the symptoms that accompany full infection help the disease spread. Runny nose, coughing and sneezing all spread virus particles and contaminate other surfaces. It's common sense that while the symptoms are in full swing the flu is most contagious.

The length of the contagious period, based on the prior reasoning, depends on how long the body's immune system takes to respond and how long the symptoms last. For healthy individuals, the symptoms last 5-7 days after they appear. However, groups with weaker immune systems including immuno-compromised individuals, the very young, and the elderly will generally have an infectious period that can persist for up to two weeks.

Combining the minor contagious period immediately following infection and the time the virus spends in the system, most healthy individuals are contagious for about 8 days after the initial infection. This is about one day before symptoms appear and another week after.

References

CDC Influenza Page

NY Department of Health - Influenza Fact Sheet

Learn more about this author, Joshua Horn.
Click here to send this author comments or questions.