Influenza viruses infecting cells

How the flu virus affects the body

What happens when the influenza virus invades?

The impact of an influenza virus depends on viral and host factors. Previous infections or vaccination should provide some protection. If the virus is novel, the severity of an infection will depend on its haemagglutinin, neuraminidase and other viral proteins, which enable the virus to reproduce better in human cells.

In fact, tissue damage has two origins – cells killed by the virus and damage caused to the body’s own cells by the immune system. Both Spanish flu and H5N1 avian flu replicate to high levels and provoke exceptionally powerful immune responses, which run away out of control in the lungs. Influenza viruses also leave patients at risk of secondary bacterial infections.

Influenza A(H1N1)pdm09 caused comparatively mild symptoms in most people. Those who died usually (but not always) had underlying health problems, such as asthma or other respiratory ailments, morbid obesity or diabetes. Pregnant women were also at higher risk.

Flu pandemics typically affect younger age groups than seasonal flu. Influenza A(H1N1)2009pdm initially had less impact on older groups, possibly because they retained immunity from earlier H1N1 epidemics or benefited from some crossover immunity from vaccination. As the virus adapted to humans, however, it began to infect adults more often.

Infants and young children may be at risk because their immune systems are immature. Paradoxically, young adults may be at risk because they have the strongest immune systems.

Genetic susceptibility

The 2009 pandemic provided a unique opportunity to explore whether particular host genetic factors increased the risk of severe disease. Indeed, people with severe disease were more likely to have a particular allele of the IFITM3 gene, which experimental studies have suggested might influence susceptibility to infection.

IFITM3 appears to play a role in preventing the influenza virus from infecting a cell and replicating. A better understanding of natural mechanisms of antiviral defence could suggest new opportunities for antiviral drug development.

Lead image:

Influenza viruses (blue) attaching to the cells of the upper respiratory tract. Viruses floating in the air are breathed in and bind to the hair-like microvilli and cilia on the surface of the cells that line the trachea. They then enter the cells and start to proliferate, eventually causing the cells to die.

R Dourmashkin/Wellcome Images CC BY NC ND


About this resource

This resource was first published in ‘Influenza special issue’ in October 2009 and reviewed and updated in January 2015.

Physiology, Health, infection and disease, Immunology
Influenza special issue
Education levels:
16–19, Continuing professional development