Influenza pandemics occur when a new strain of the influenza virus is transmitted to humans from another animal species. Species that are thought to be important in the emergence of new human strains are pigs, chickens and ducks. These novel strains are unaffected by any immunity people may have to older strains of human influenza and can therefore spread extremely rapidly and infect very large numbers of people. Influenza A viruses can occasionally be transmitted from wild birds to other species, causing outbreaks in domestic poultry, and may give rise to human influenza pandemics.^[1][2] The propagation of influenza viruses throughout the world is thought in part to be by bird migrations, though commercial shipments of live bird products might also be implicated, as well as human travel patterns. 

The World Health Organization (WHO) has produced a six-stage classification that describes the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. This starts with the virus mostly infecting animals, with a few cases where animals infect people, then moves through the stage where the virus begins to spread directly between people, and ends with a pandemic when infections from the new virus have spread worldwide.

You can protect yourself and help prevent spreading the virus to others if you:

Do

Wash your hands regularly for 20 seconds, with soap and water or alcohol-based hand rub

Cover your nose and mouth with a disposable tissue or flexed elbow when you cough or sneeze

Avoid close contact (1 meter or 3 feet) with people who are unwell

Stay home and self-isolate from others in the household if you feel unwell

Don't

Touch your eyes, nose, or mouth if your hands are not clean

Coronavirus causes a respiratory disease called COVID-19. Symptoms include fever, cough, and shortness of breath. Most cases (about eight out of ten) are mild. The more serious cases can lead to severe lung damage.

Coronavirus is thought to spread mostly through close contact. An infected person coughs or sneezes, sending droplets containing the virus through the air. Someone nearby – say, up to about six feet away – breathes those droplets and can become infected. The virus can also spread hand-to-hand. You touch an object that has the virus on it, then touch your mouth, nose, perhaps even your eye. Scientists are studying how long the coronavirus can survive on various surfaces or whether there are other ways of spread.