Introduction
Influenza viruses are classified as type A, B, or C based upon their protein composition. Type A viruses are found and can cause disease in many kinds of animals, including ducks, chickens, pigs, whales, and also in humans. The type B virus widely circulates and causes disease in humans only. Type C has been found in humans, pigs, and dogs and causes mild respiratory infections, but does not spark epidemics. Type A influenza is the most frightening of the three and is responsible for global influenza outbreaks in 1918, 1957 and 1968.
Influenza A (flu) is a roughly round (A), enveloped virus. It can also be elongated filamentous or irregularly shaped (B). Inside are eight segments of single, negative -strand RNA containing the genetic instructions for making new copies of the virus. Flu's most visually striking feature is a layer of projections on its surface. There are two different types of projections: one is the protein hemagglutinin (HA), which allows the virus to "stick" to a cell and initiate infection, the other is a protein called neuraminidase (NA), which enables newly formed viruses to exit the host cell.
Figure A and B show typical morphology and filamentous morphology. Figure C shows the glycoproteins (spikes) and segmented genome. (Electron micrographs from the CDC public health image library)
Figure A
Figure B
Figure C
