Diseases are described by how widely they occur:
• Endemic: always present at a steady level in a particular region, e.g. malaria in parts of Africa.
• Epidemic: a sudden rise in the number of cases, well above normal, in one region or country.
• Pandemic: an epidemic that spreads across several countries or continents, affecting large numbers of people worldwide.Pandemics are nearly always caused by a new strain of virus to which almost nobody has immunity. New strains arise in two main ways:Antigenic drift: small mutations gradually change the viral antigens. This causes the regular seasonal outbreaks of flu.Antigenic shift: a sudden, large change. In influenza, if an animal (such as a pig) is infected by two different strains at once, whole RNA segments can be swapped, producing a completely new combination of antigens. Most people's memory cells do not recognise it.New viruses can also jump from animals to humans, which is called zoonotic spillover. HIV came from chimpanzees and SARS-CoV-2 probably from bats.A new virus spreads worldwide quickly because of international air travel, dense cities and people who are infectious before they have symptoms.Examples of pandemics include:
• the 1918 influenza pandemic, which killed an estimated 50 million people
• HIV/AIDS, from the 1980s onward
• the 2009 H1N1 ("swine flu") influenza pandemic
• COVID-19, declared a pandemic by the World Health Organization (WHO) in March 2020The WHO coordinates the global response, sharing surveillance data and advice on control measures.
• Endemic: always present at a steady level in a particular region, e.g. malaria in parts of Africa.
• Epidemic: a sudden rise in the number of cases, well above normal, in one region or country.
• Pandemic: an epidemic that spreads across several countries or continents, affecting large numbers of people worldwide.Pandemics are nearly always caused by a new strain of virus to which almost nobody has immunity. New strains arise in two main ways:Antigenic drift: small mutations gradually change the viral antigens. This causes the regular seasonal outbreaks of flu.Antigenic shift: a sudden, large change. In influenza, if an animal (such as a pig) is infected by two different strains at once, whole RNA segments can be swapped, producing a completely new combination of antigens. Most people's memory cells do not recognise it.New viruses can also jump from animals to humans, which is called zoonotic spillover. HIV came from chimpanzees and SARS-CoV-2 probably from bats.A new virus spreads worldwide quickly because of international air travel, dense cities and people who are infectious before they have symptoms.Examples of pandemics include:
• the 1918 influenza pandemic, which killed an estimated 50 million people
• HIV/AIDS, from the 1980s onward
• the 2009 H1N1 ("swine flu") influenza pandemic
• COVID-19, declared a pandemic by the World Health Organization (WHO) in March 2020The WHO coordinates the global response, sharing surveillance data and advice on control measures.