FactSentinel

Seasonal influenza is caused by viruses, not bacteria. The culprits are influenza viruses, primarily types A and B, which infect cells in the nose, throat, and lungs. Viruses and bacteria are fundamentally different organisms. Bacteria are single-celled living organisms that can reproduce on their own, while viruses are much smaller particles that must hijack a host's cells to replicate. The influenza virus was first isolated in 1933, and modern laboratory methods such as PCR testing and rapid antigen tests identify flu by detecting viral genetic material or proteins, not bacterial ones.

This distinction matters for treatment and prevention. Antibiotics work by targeting bacterial structures and processes, so they have no effect on influenza. Flu is instead managed with rest, fluids, and in some cases antiviral medications such as oseltamivir, which block viral replication. Annual flu vaccines are reformulated each year to match circulating virus strains, which change frequently through genetic mutation. Public health agencies such as the CDC and the World Health Organization are clear on the viral cause of the disease.

The confusion likely stems from a few sources. Flu symptoms can resemble those of bacterial illnesses, and influenza can sometimes lead to secondary bacterial infections, such as bacterial pneumonia, which do require antibiotics. Historically, a bacterium called Haemophilus influenzae was mistakenly blamed for causing flu during the 1918 pandemic, which is reflected in its name. Scientists later established that it is not the cause of influenza.