More about influenza

This page provides links to PATH's work on influenza and information about the disease and related vaccines.

On the PATH website

Influenza disease

  • Influenza, or flu, can be caused by three different virus types: influenza A, influenza B, and influenza C.
  • Influenza occurs in seasonal patterns worldwide and is estimated to cause three to five million cases and 250,000 to 500,000 deaths each year. The most severe influenza pandemic, in 1918, infected up to 50 percent of the world’s population and caused 20 to 50 million deaths worldwide.
  • The most recent influenza pandemic occurred in 2009 wherein a new strain of influenza A, pandemic A (H1N1) 2009, emerged, spread rapidly around the world, and caused sustained community-level outbreaks. Currently available data suggest that the overall severity of the influenza pandemic was moderate; however, that does not mean that morbidity and mortality were not substantial.
  • Influenza symptoms include fever, headache, cough, tiredness, sore throat, runny or stuffy nose, body aches, and diarrhea or vomiting. These symptoms are common to several illnesses, which makes it harder to determine the incidence of influenza, particularly in countries where a doctor is not readily accessible.
  • The most common method of influenza transmission is human-to-human, although animal-to-human transmission is possible. A person may infect others even though they seemingly have no influenza symptoms.
  • Up to now, one of the deadliest influenza strains, avian influenza A (H5N1), is fortunately not easily spread from human-to-human; however, public health leaders are following the virus closely to see if it will evolve into a form that spreads efficiently from human to human.
  • Influenza can be treated with antiviral drugs, which can lessen the severity of disease and are effective against influenza virus types A and B.

Influenza vaccines

  • Each year, currently circulating influenza strains are tested to determine the three strains that will be included in the vaccine, most commonly made from embryonated chicken eggs. Usually, one to two strains covered by the vaccine change each year.
  • The influenza vaccine (using inactivated influenza virus) is approved for use among healthy people six months of age or older and those with chronic medical conditions (such as asthma, diabetes, or heart disease). The intranasal, attenuated, live-virus vaccine, FluMistĀ®, is recommended for healthy individuals between 2 and 50 years old who are not pregnant.
  • Past experience shows that high-resource countries may experience mild disease from influenza, but more severe disease may occur, with higher mortality, in developing countries. New options for influenza vaccine development are an important component of pandemic preparedness for both poor countries and the world community.
  • Rapid production and distribution of pandemic influenza vaccines could potentially save millions of lives during an influenza pandemic. Preparatory efforts are under way to advance technologies that can be produced more easily and affordably than current vaccines and do not rely upon egg-based technology, which could be compromised in the event of an avian-strain pandemic.
  • Investigating new vaccine technologies will benefit real-time response in a possible future pandemic, but there is no immediate solution yet.

Related diseases

There are a number of repiratory diseases, all of which affect the lungs and airways, including pneumococcus, respiratory syncytial virus (RSV), Haemophilus influenzae type b (Hib), and influenza. Learn more about pneumococcus. Learn more about RSV. Learn more about Hib.


Page last updated: February 2012.