How Long is the Flu Contagious?

Influenza is usually the most contagious in the first three or four days after the illness begins. However, the beginning of the illness does not necessarily coincide with noticeable symptoms. Most adults are capable of spreading the flu a full 24 hours before symptoms develop. Symptoms usually develop one to four days after the influenza virus enters the body.

Once the virus enters the body, it may be contagious for up to a week. However, symptoms may last for up to 10 days. One sign that indicates someone is no longer contagious is a fever. If a person is fever-free for at least 24 hours without taking any fever-suppressing medication, they are likely no longer contagious.

Influenza viruses can spread using many different methods. Though uncommon, some people contract the flu by touching a surface or object that has the virus on it. They then touch their mouth, nose, or eyes and the virus enters the body. Research on influenza viruses shows that the virus can survive on non-porous materials such as metal for at least 24 hours. If the virus is present in mucus on a surface, it may stay active for up to 17 days.

The most common method for the spread of influenza is direct person-to-person contact. Whenever an infected person speaks, coughs, or sneezes, they expel more than half a million virus particles into the air. These droplets can land in another person’s mouth, nose, or eyes and infect them. Transmission rates are highest while within six feet of an infected individual.

For many reasons, children are the most susceptible to the flu and the most likely to spread it. Children under 18 are more than twice as likely to develop symptomatic flu as adults over 65. One reason for this is that children have a longer period of viral shedding before symptoms begin. Children under five also have a longer period of viral shedding after symptoms end. As long as viral shedding continues, a person is contagious.

Like children, immunocompromised people may be contagious for a significantly longer period than someone with a healthy immune system. One study showed that viral shedding can continue for longer than two weeks in immunocompromised individuals. Because people with immune system issues are also more likely to contract the virus and have worse symptoms, prevention is key.

The CDC recommends several methods to lower the risk of contracting the flu. Their primary suggestion is that everyone over the age of six months, with a few exceptions, receive a flu vaccine by the end of October each year. These vaccines protect against the four strains of influenza that are most likely to spread that year. However, it is possible to contract the flu two weeks before and after receiving a vaccine.

To further decrease the likelihood of getting the flu, it is integral to practice everyday preventative measures. Cover coughs and sneezes with a tissue, mask, or elbow. Wash hands regularly, particularly after coughing or sneezing. Avoid touching the eyes, nose, and mouth. Additionally, do not interact with someone who is sick.

In the United States, the fall and winter encompass "flu season." This is the period where flu cases increase dramatically. Generally, the highest peaks are between December and February. Though rare, past flu seasons have continued well into May. During these months, it is important to practice proper prevention measures.

Most people have mild cases and do not need treatment for their flu. However, people in high-risk groups or with severe cases may receive antiviral medications, prescriptions that can make the flu milder and shorten the length of the illness. Antiviral drugs are most effective when taken within two days after contracting the flu.