Pandemics can be prolonged, continuing for many months or for over a year. The impact of a pandemic is highly variable but can be very widespread, affecting many areas of daily life. Because the human population has little or no immunity to the disease, it can spread rapidly across the globe and may result in high numbers of cases and deaths.
The impact of a pandemic depends on how sick the pathogen makes people (clinical severity), the ability of the pathogen to spread between people (transmissibility), the capacity of the health system, the effectiveness of interventions and the vulnerability of the population.
A pandemic can be due to a new or re-emergent disease that spreads between countries and continents.
Examples are the most recent COVID-19, mosquito-borne diseases such as Zika, new respiratory viruses such as SARS (severe acute respiratory syndrome) and MERS (Middle eastern respiratory syndrome) and viral haemorrhagic infections such as Ebola.