The World Health Organisation has repeatedly stopped short of calling the coronavirus outbreak a pandemic – until recently. Speaking at a press conference on Wednesday 11 March the director general, Dr Tedros Adhanom Ghebreyesus, finally used the term to describe the outbreak, which has now spread to well over 100 countries and infected over 185,000 people. "WHO has been assessing this outbreak around the clock and we are deeply concerned both by the alarming levels of spread and severity, and by the alarming levels of inaction," Dr Tedros said. "We have therefore made the assessment that Covid-19 can be characterized as a pandemic. Pandemic is not a word to use lightly or carelessly," he added. But what does this actually mean, is it a big deal and what are the implications? What is a pandemic? Pandemic, a word from the Greek pan ("all") and demos ("people"), is the term used by disease experts when epidemics are growing in multiple countries and continents at the same time. This is different to an epidemic – which is usually used to describe an outbreak that has grown out of control yet is limited to one just country or location. But despite the fear the word evokes, "pandemic" refers to the spread of a disease, not its potency or deadliness. The WHO defines the term as "an outbreak of a new pathogen that spreads easily from person to person across the globe". Read More: has coronavirus hit my area?

This means a disease outbreak will be labelled as a pandemic when it is widespread, over several countries or continents, usually affecting a large number of people. The disease must also be infectious – cancer affects many people around the world, but it is not infectious and so is not a pandemic. A pandemic also has self-sustaining lines of infection – which means that human to human transmission is widespread. Until now, the WHO has stopped short of calling the outbreak a pandemic because they said that local spread was limited and most cases had a connection to China or another emerging hotspot – for instance Iran or Italy. But it's now clear that local transmission is incredibly widespread, with over 115 countries detecting the virus and more than 10 confirming at least 500 cases. When was the last pandemic declared? Pandemics can vary greatly in scale and potency – there is no set threshold, such as cases or deaths, that have to be met. But past examples include HIV, swine flu and the 1918 Spanish flu. Some of the most deadly pandemics in history were the Black Death, which killed up to 200 million people in the Middle Ages, and smallpox, which killed about 300 million in the 20th century. But the last time the WHO declared a pandemic was in 2009 for the H1N1 influenza outbreak. At the time, the decision was criticised by some countries which felt that it caused unnecessary panic. It also led to many nations wasting money on vaccines for a strain of flu that proved to be mild and relatively easy to contain. What is the process for announcing a pandemic? But a lot has changed since 2009 – including WHO processes. The organisation says there is no longer a formal process to categorise an outbreak as a pandemic – instead, they announced on January 30 that the epidemic was a public health emergency. This is the highest level classification to describe a disease outbreak under the International Health Regulations and, officially speaking, probably changed the response more than the announcement of pandemic. A spokesperson told The Telegraph: "WHO does not use the old system of six phases – that ranged from phase one (no reports of animal influenza causing human infections) to phase six (a pandemic) – that some people may be familiar with from H1N1 in 2009. "We do use term pandemic for all sorts of purposes and may qualify a situation as pandemic, but there would no official announcement." So, when Dr Tedros called the epidemic a pandemic, it was not a decision based on a formal process and there is not a set list of new interventions that will now come into force.

Tedros Adhanom Ghebreyesus, director general of the World Health Organization Credit : Stefan Wermuth/Bloomberg

But language matters and using the term is still a significant step. The WHO was keen to emphasise that there had been a lot of consultation with people both inside and outside the organisation. Mike Ryan, director of emergencies at the WHO, said: "This is not a trigger for anything other than more aggressive or intense action. It's taken very seriously and we understand the implication of the word. "The director general has gone through very complex internal and external consultation over long hours on the use of the word," he said. Does calling the outbreak a pandemic change anything? Dr Tedros also said he used the word because he was alarmed at "levels of inaction" around the globe – so it's likely that he wanted to pressure nations to take further steps to contain the coronavirus. "WHO has been assessing this outbreak around the clock and we are deeply concerned both by the alarming levels of spread and severity, and by the alarming levels of inaction," he said. But he then added: "Describing the situation as a pandemic does not change WHO’s assessment of the threat posed by this coronavirus. It doesn’t change what WHO is doing, and it doesn’t change what countries should do." "I remind all countries that we are calling on you to: activate and scale up your emergency response mechanisms; communicate with your people about the risks and how they can protect themselves; find, isolate, test and treat every Covid-19 case and trace every contact; ready your hospitals; protect and train your health workers. "Let’s all look out for each other," he said.