Covid-19 is proving exceptionally difficult to stop, and it is important to identify where the human-animal barrier was breached, Dr. Mike Ryan, executive director of the World Health Organization’s Health Emergencies Programme, said during a news conference in Geneva on Monday.

“This virus is proving exceptionally difficult to stop,” Ryan said, saying that it was important to reflect on what is known about the virus, in terms of the ease of spread, the multiple transmission modes and the existence of asymptomatic and presymptomatic transmission.

“This is not an easy virus and not an easy virus, either, to detect. It’s not an easy virus to stop,” he said.

Ryan explained that it is difficult to detect and distinguish between Covid-19 and other syndromes without adequate and immediate testing which is being seen in countries where both Covid-19 and influenza are circulating at the same time.

An epidemiologic investigation will start in Wuhan, because that’s where the first clusters of cases were picked up, he said, but that doesn’t necessarily mean that is where Case Zero was, according to Ryan.

“If you follow the data and the science, you will find, hopefully, the point at which the disease crossed the species barrier,” he said.

Finding Case Zero helps to distinguish this crossing of the barrier between species.

“It is important that we find that, because as long as the animal-human breach has not been discovered, there’s always a chance that that barrier can be breached again,” he said.

Although finding this breach is important, it can take some time, according to Ryan, who said It took years for MERS, and has never been fully established for SARS.

The world is at greater risk for these species breaches, Ryan said.

“We are pressuring the biologic system. We live in a biome. We live in a world of biology. And we are creating – actively creating – the pressures that are driving the breaches of those barriers,” he said. “And we need to do better at managing the risks associated with that.”