A pandemic is a worldwide epidemic of some highly contagious infectious disease. Pandemics often occur in two or three distinct waves, the second of which is usually the most deadly. If there is a third wave, it is usually the least deadly of the three.
Influenza and Its Causes
Influenza, or the flu, is a respiratory illness caused by a group of related-Ribonucleic Acid (RNA) viruses. Influenza viruses are divided into types A, B, and C. Influenza type A and type B viruses cause outbreaks of the flu in humans throughout the year. Influenza A viruses are further divided into subtypes based on two proteins on the surface of the virus. These proteins are called hemagglutinin (H) and neuraminidase (N), this the H-N classification system. Wild birds are the primary natural reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza A viruses in all other animals. Both avian and human influenza viruses can infect swine.
Influenza A occurs worldwide throughout the year and in the United States alone, accounts for approximately 36,000 deaths and 200,000 hospitalizations each year. Most fatalities during normal flu season are among the very old, very young, debilitated, and those with compromised immune systems. About every 30 years, or three times a century, an outbreak of influenza occurs with a virus that is new to the human immune system, resulting in the rapid worldwide spread of the disease and causing, by definition, a pandemic. In about one third of these outbreaks, a virus emerges that is particularly virulent, contagious, and lethal. The great pandemics cause by these unusual strains of influenza are characterized by a number of distinct differences from the annual flu outbreaks. These exceptionally lethal viruses carry mortality rates up to 10 times those of the more common epidemics, but, more importantly, they seem to attack young healthy individuals in addition to the old and infirm. To further compound the problem, the mortality rates tend to be higher among these younger and more healthy victims than among the old and debilitated. The immune response elicited in the younger, previously healthy patients is so extremely vigorous that the patient's own immune response actually results in his or her rapid deterioration and death.