influenza commonly called "the flu " is an illness caused by rna viruses that infect the respiratory tract of many animals birds and humans. in most people the infection results in the person getting fever cough headache and malaise (tired no energy); some people also may develop a sore throat nausea vomiting and diarrhea. the majority of individuals has symptoms for about one to two weeks and then recovers with no problems. however compared with most other viral respiratory infections such as the common cold influenza (flu) infection can cause a more severe illness with a mortality rate (death rate) of about 0. 1% of people who are infected with the virus. the above is the usual situation for the yearly occurring "conventional" or "seasonal" flu strains. however there are situations in which some flu outbreaks are severe. these severe outbreaks occur when the human population is exposed to a flu strain against which the population has little or no immunity because the virus has become altered in a significant way. unusually severe worldwide outbreaks (pandemics) have occurred several times in the last hundred years since influenza virus was identified in 1933. by an examination of preserved tissue the worst influenza pandemic (also termed the spanish flu or spanish influenza) occurred in 1918 when the virus caused between 40-100 million deaths worldwide with a mortality rate estimated to range from 2%-20%. in april 2009 a new influenza strain against which the world population has little or no immunity was isolated from humans in mexico. it quickly spread throughout the world so fast that the who declared this new flu strain (termed novel h1n1 influenza a swine flu often shortened to h1n1 or swine flu) as the cause of a pandemic on june 11 2009. this was the first declared flu pandemic in 41 years. fortunately there was a worldwide response that included vaccine production good hygiene practices (especially hand washing) were emphasized and the virus (h1n1) caused far less morbidity and mortality than was expected and predicted. the who declared the pandemic's end on aug. 10 2010 because it no longer fit into the who's criteria for a pandemic. haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until the virus was demonstrated to be the correct cause in 1933. this bacterium can cause lung infections in infants and children and it occasionally causes ear eye sinus joint and a few other infections but it does not cause the flu.
what is influenza?