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What is a pandemic?

A pandemic is an epidemic of disease that has spread across multiple countries and continents, or worldwide. An epidemic is an outbreak of disease in a population, well above the levels normally seen in that area, affecting a large number of people.

When a disease is continuously present in an area, it is said to be endemic.

Relatively few diseases become true, global pandemics. One example is HIV/AIDS, which spread within a few years to affect millions of people in every country in the world.

Most areas of the world see seasonal increases in the number of cases of influenza. Some year’s outbreaks are much more severe than others. When the influenza season is particularly severe it is considered a pandemic. For example the “Spanish flu” outbreak of 1918-20 is estimated to have killed about 50 million people worldwide.

Six Phases of a Pandemic

To aid in preparedness and planning, the World Health Organization defines six phases of pandemic influenza. This framework is a planning tool: phases may not proceed in order.

Phase 1: Influenza viruses circulate in wild or domestic animals with no infections in humans.

Phase 2: Virus from animals causes infection in humans and is a possible pandemic threat.

Phase 3: Sporadic outbreaks in humans, but no sustained transmission of virus directly between people.

Phase 4: Sufficient person-to-person spread to cause community-level outbreaks. This marks an increase in the risk of a pandemic.

Phase 5: Person-to-person spread in at least two countries within one WHO region.

Phase 6: As for phase 5, plus at least one outbreak in a country in a different WHO region. This constitutes a global influenza pandemic.

What is the Ebola virus?

In 1976, Ebola Virus Disease erupted in two nearly simultaneous outbreaks in South Sudan and in the Democratic Republic of Congo, by the Ebola river. Nearly 600 cases were reported during that year. Since then, Central Africa in particular has experienced periodic outbreaks of Ebola virus disease, formerly called Ebola hemorrhagic fever.

What we call the “Ebola virus” is actually five virus strains. Ebola is a classic example of a zoonotic infection that circulates among wild animals and occasionally “spills over” into humans. The natural host of Ebola virus is unknown, but closely related viruses have been discovered in bats. Humans acquire it when they come in contact with an infected animal, perhaps through hunting.

Ebola viruses spread through direct contact with bodily fluids from infected individuals or indirect contact with contaminated surfaces. Once a patient is infected, Ebola is difficult to recognize and diagnose. Symptoms are similar to other common illnesses and manifest between eight to ten days after contact.

The largest, deadliest outbreak occurred in 2014, totaling over 26,000 cases throughout West Africa and a handful worldwide. About half of these cases were fatal.

A number of vaccines against Ebola virus are under development or being tested in clinical trials. These vaccines appear promising but it is not yet clear how effective they are in protecting against illness. Nonetheless, health organizations have recently used experimental vaccines in a “ring vaccination” strategy to protect people at risk of exposure to Ebola, such as the family members of patients.