Influenza is a respiratory illness caused by viruses. It is widespread and can lead to symptoms ranging from mild to severe. Understanding its spread is important for public health and limiting its impact.
Direct Spread from Person to Person
The flu virus primarily spreads through respiratory droplets expelled when an infected individual coughs, sneezes, or talks. These droplets can travel approximately six feet before settling. Infection occurs if these virus-laden droplets land on another person’s mucous membranes, such as the mouth, nose, or eyes.
Smaller particles, known as aerosols, also contribute to airborne spread. Unlike larger droplets, aerosols can remain suspended in the air for minutes or hours and travel further distances. Aerosols can lead to infection, particularly in enclosed spaces with limited airflow.
Indirect Spread Through Surfaces
The flu virus can also spread indirectly through contaminated surfaces. When an infected person coughs or sneezes, respiratory droplets can land on objects like doorknobs, phones, or shared items. The virus can remain infectious on hard, non-porous surfaces, such as stainless steel and plastic, for 24 to 48 hours.
On porous surfaces like cloth, paper, and tissues, the virus survives for less than 8 to 12 hours. If a person touches a contaminated surface and then touches their mouth, nose, or eyes, the virus can transfer to their body. The virus’s infectiousness on hands is limited to about five minutes after transfer from a surface.
When People Are Contagious
An individual infected with the flu can begin transmitting the virus before symptoms appear, starting about one day before illness onset. Symptoms develop about two days after exposure to the virus. People are most contagious during the first three to four days after their symptoms begin.
Spread can persist for up to five to seven days after symptoms start. Children and individuals with weakened immune systems may remain contagious for a longer duration. Children can spread the virus for 7 to 10 days, and immunocompromised individuals for several weeks.
How Environment Affects Flu Transmission
Environmental conditions play a role in how the flu virus spreads and survives. Temperature and humidity influence the stability and infectiousness of viral particles. The virus survives longer and transmits more effectively in colder temperatures and lower humidity. Cold, dry conditions help viral particles remain stable and airborne for longer periods.
Ventilation also affects flu transmission. Poorly ventilated spaces allow airborne virus particles to accumulate, increasing transmission risk. Introducing fresh outdoor air helps to dilute and remove virus particles from indoor spaces.