Coronavirus cases have fallen dramatically in recent weeks in many northern hemisphere countries.
But can all the reduction be due to lockdown measures and social distancing? Here is what we know about the effect of climate, sunshine and vitamin D on coronavirus.
Latest research suggests coronavirus is following a very specific path around the globe, leaving some countries unscathed, while having a disproportionately devastating impact elsewhere.
The University of Maryland found that most cases fall along a narrow east-west corridor of 30 and 50 degrees of latitude, which includes northern Italy, the Pacific north-west, Japan, Iran, South Korea, France, Spain and Germany. All share similar climates.
In contrast, areas expected to be hardest hit, because of geographical proximity and travel connections to the Chinese outbreak - such as south-east Asia - have had low infections and deaths compared to those in the "coronavirus belt".
None of the temperatures in badly affected cities dipped below 0C during the height of the epidemic, which suggests a threshold beyond which the virus cannot survive.
The University of Oxford also conducted a review into whether climate was playing a role in the transmission of coronavirus, and found that cold and dry conditions appeared to boost its spread. It found that while the global death rate was 0.2pc, in the northern hemisphere it was 0.3pc and even discovered a gradient relationship in Italy with the south being less affected than the north.
Analysis of the previous Sars outbreak in Hong Kong has shown the number of daily cases was higher on days where the weather was cooler and research has shown viruses can live longer on surfaces when the weather is cold.
There is growing evidence that vitamin D is protective against coronavirus and that people with chronically low levels may be at greater risk.
Researchers from the Anglia Ruskin University compared the numbers of coronavirus cases to the average levels of vitamin D for 20 European countries and found a significant correlation.
Italy and Spain have both experienced high mortality rates, and scientists found both countries have lower than average vitamin D levels.
This is partly because people in southern Europe, particularly the elderly, avoid strong sun, while their darker skin pigmentation also reduces the body's ability to produce natural vitamin D.
The highest average levels of vitamin D are found in northern Europe, due to high consumption of cod liver oil and vitamin D supplements, and possibly less sun avoidance.
Scandinavian nations are among the countries with the lowest number of Covid-19 cases and mortality rates per head of population in Europe.
A study by Trinity College Dublin published this week also shows vitamin D appears to help reduce serious complications with coronavirus.
During the Spanish flu pandemic of 1918, doctors noticed patients who were nursed outdoors appeared to fare better.
Sunlight is known to be germicidal and there is growing evidence it can kill viruses too.
Viruses tend to survive better in cold weather because they have a fatty coating that degrades when it is warm.
Prof Keith Neal, emeritus professor in the epidemiology of infectious diseases at the University of Nottingham, said: "Sunlight includes ultraviolet radiation. This damages DNA and RNA. I have not seen any work on how quickly this affects Covid-19 but viruses left on surfaces outside will dry out and be damaged by UV light in sunlight."
Open-air therapy was once a popular treatment for deadly respiratory conditions such as tuberculosis, and patients were regularly put outside until antibiotics became standard in the 1950s.
Hospital wards were once "cross-ventilated" with large windows to allow fresh air to move freely, but modern hospitals mostly have closed air-conditioning systems.
Prof Neal said fresh air quickly dispersed any virus droplets in the atmosphere.
"Talking and coughing can produce droplets and aerosols. Droplets, which are larger than aerosols, carry more virus but fall rapidly to the ground under gravity," he said. (© Daily Telegraph, London)