Pneumonia can be caused by bacteria, viruses, or fungi. Bacterial generally cause the most severe disease. The worst case is bacterial pneumonia that occurs at the same time as, or shortly after, an influenza infection. Dr. Koenraad F van der Sluijs and colleagues at the Academic Medical Center, Amsterdam, The Netherlands, provided a review.
The most prominent bacteria that causes pneumonia is Streptococcus pneumoniae. Other important pneumonia-causing bacteria are Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pyogenes. “Individuals infected with influenza virus are most susceptible to secondary bacterial pneumonia between 4 and 14 days after the onset of influenza symptoms,” van der Sluijs reported.
Van der Sluijs explained that influenza infection damages the airways and leaves them more vulnerable to bacterial infection. “The severity of post-influenza pneumonia is due to virus-induced changes to the host that affect the course of bacterial infection.”
Historically, secondary bacterial pneumonia was believed to be responsible for most of the deaths from the 1918 “Spanish” influenza pandemic. During the later influenza pandemic of 1957, more than two-thirds of deaths were associated with bacterial pneumonia. Prompt antibiotic treatment is required to reduce mortality.
Dr. Richard G. Wunderink of Northwestern University Feinberg School of Medicine, Chicago, Illinois, published a comment on van der Sluijs' paper. It is difficult to determine how many deaths are due to viral infection alone, bacterial infection alone, or a combination, he reported. He recommended more use of vaccinations and antiviral drugs, as well as antibiotic treatment.
The influenza virus has two major types that affect humans, A and B. Influenza B has generally been considered less pathogenic, with little risk of death, according to a case report by Dr. Timothy Aebi and colleagues in Switzerland. However, these authors identified Influenza B infection in three previously healthy women who were treated in their hospital for pneumonia and septic shock.