OPINION: There are now over 24 million confirmed cases and I’ve been thinking about how this pandemic compares to other major outbreaks over the last 20 years. What makes Covid-19 different from Sars, H1N1, Ebola and Zika?

Sars appeared in late 2002, also caused by a coronavirus that spreads through the respiratory route. Unlike Covid-19, people with Sars had a high fever early in their infection. That made it easier to identify infected people and stop human-to-human transmission. If only the same was true for Covid-19.

By mid-2004, Sars was gone and hasn’t been seen since. By then 8000 people had been infected and over 800 had died. Cases had spread to almost 30 countries and territories. Covid-19 also emerged in a globally connected part of the world and at a time of year when lots of people were moving about.

H1N1 was a variant of the influenza viruses from humans, birds, and pigs that caused a pandemic from early 2009 to August 2010. Like normal seasonal flu, H1N1 spread through the respiratory route. But unlike normal flu, it was more likely to cause breathing difficulties in young, healthy people. Thankfully, a vaccine was available by late 2009. It’s thought H1N1 caused about 500,000 deaths. We’re already over 800,000 confirmed deaths for Covid-19.

The largest Ebola outbreak began in Guinea, West Africa in December 2013 and spread to Liberia and Sierra Leone. Ebola transmits through bodily fluids from symptomatic people. That means it’s easier to stop than Covid-19, in which people are infectious before they realise they have the virus.

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While vaccines were in clinical trials by mid-2015, the Ebola outbreak was mainly brought under control by stopping human-to-human transmission. It also helped that it was in a part of the world that isn’t quite so globally connected. The outbreak was officially declared over in June 2016. By then over 28,000 people had been infected and over 11,000 had died.

Zika is the virus that causes babies to be born with small heads. It’s spread by mosquito bite and caused an outbreak in the Americas, Pacific, and Southeast Asia in 2015 and 2016. In many mosquito species, the females feed on people one time before laying their eggs. Zika is carried by mosquitoes that feed more than once. As a result, they spread the virus from infected to uninfected people as they ate. The outbreak was largely controlled by getting rid of mosquitoes carrying the virus.

I think it helps to think of these outbreaks and pandemics as a handful of dice. One represents the microbe and how it spreads. Another, what symptoms it causes. Then there is a dice for where and when the disease emerged. And finally, another for how it can be treated and prevented. How each dice falls influences how the outbreak plays out.

With Covid-19, we’ve rolled almost the worst possible combination, with a collection of ones when what we really needed were a mix of fives and sixes.