The coronavirus has killed so many people in Iran that the country has resorted to mass burials, but in neighboring Iraq, the body count is fewer than 100.
The Dominican Republic has reported nearly 7,600 cases of the virus. Just across the border, Haiti has recorded about 85.
In Indonesia, thousands are believed to have died of the coronavirus. In nearby Malaysia, a strict lockdown has kept fatalities to about 100.
The coronavirus has touched almost every country on earth, but its impact has seemed capricious. Global metropolises like New York, Paris, and London have been devastated, while teeming cities like Bangkok, Baghdad, New Delhi, and Lagos have, so far, largely been spared.
The question of why the virus has overwhelmed some places and left others relatively untouched is a puzzle that has spawned numerous theories and speculations but no definitive answers. There are already hundreds of studies underway looking into how demographics, preexisting conditions, and genetics might affect the wide variation in impact.
Many developing nations with hot climates and young populations have escaped the worst, suggesting that temperature and demographics could be factors. But countries like Peru, Indonesia, and Brazil, tropical countries in the throes of growing epidemics, throw cold water on that idea.
Draconian social distancing and early lockdown measures have clearly been effective, but Myanmar and Cambodia did neither and have reported few cases.
One theory that is unproven but impossible to refute: Maybe the virus just hasn’t gotten to those countries yet. Russia and Turkey appeared to be fine until, suddenly, they were not.
Interviews with more than two dozen infectious disease specialists, health officials, epidemiologists, and academics around the globe suggest four main factors that could help explain where the virus thrives and where it doesn’t: demographics, culture, environment, and the speed of government responses.
Each possible explanation comes with considerable caveats and confounding counterevidence. If an aging population is the most vulnerable, for instance, Japan should be at the top of the list. It is far from it. Nonetheless, these are the factors that specialists find the most persuasive.
Many countries that have escaped mass epidemics have relatively younger populations.
Young people are more likely to contract mild or asymptomatic cases that are less transmissible to others, said Robert Bollinger, a professor of infectious diseases at the Johns Hopkins School of Medicine. And they are less likely to have certain health problems that can make COVID-19, the disease caused by the coronavirus, particularly deadly, according to the World Health Organization.
Africa — with about 45,000 reported cases, a tiny fraction of its 1.3 billion people — is the world’s youngest continent, with more than 60% of its population under age 25. In Thailand and Najaf, Iraq, local health officials found that the 20-to-29 age group had the highest rate of infection but often showed few symptoms.
By contrast, the national median age in Italy, one of the hardest-hit countries, is more than 45. The average age of those who died of COVID-19 there was around 80.
Younger people tend to have stronger immune systems, which can result in milder symptoms, said Josip Car, an expert in population and global health at Nanyang Technological University in Singapore.
Cultural factors, like the social distancing that is built into certain societies, may give some countries more protection, epidemiologists said.
In Thailand and India, where virus numbers are relatively low, people greet each other at a distance. In Japan and South Korea, people bow, and long before the coronavirus arrived, they tended to wear face masks when feeling ill.
In much of the developing world, the custom of caring for the elderly at home leads to fewer nursing homes, which have been tinder for tragic outbreaks in the West.
However, there are notable exceptions to the cultural-distancing theory. In many parts of the Middle East, such as Iraq and the Persian Gulf countries, men often embrace or shake hands on meeting, yet most are not getting sick.
The geography of the outbreak — which spread rapidly during the winter in temperate-zone countries like Italy and the United States and was virtually unseen in warmer countries such as Chad or Guyana — seemed to suggest that the virus did not take well to heat. Other coronaviruses, such as ones that cause the common cold, are less contagious in warmer, moist climates. But researchers say the idea that hot weather alone can repel the virus is wishful thinking.
Some of the worst outbreaks in the developing world have been in places like the Amazonas region of Brazil.
The virus that causes COVID-19 appears to be so contagious as to mitigate any beneficial effect of heat and humidity, said Dr. Raul Rabadan, a computational biologist at Columbia University.
But other aspects of warm climates, like people spending more time outside, could help.
The ultraviolet rays of direct sunlight inhibit this coronavirus, according to a study by ecological modelers at the University of Connecticut. So surfaces in sunny places may be less likely to remain contaminated, but transmission usually occurs through contact with an infected person, not by touching a surface.
Countries that locked down early, like Vietnam and Greece, have been able to avoid out-of-control contagions, evidence of the power of strict social distancing and quarantines to contain the virus.
In Africa, countries with bitter experience with killers like HIV, drug-resistant tuberculosis and Ebola knew the drill and reacted quickly.
Airport staff from Sierra Leone to Uganda were taking temperatures (since found to be a less effective measure) and contact details and wearing masks long before their counterparts in the United States and Europe took such precautions.
Counterintuitively, some countries where authorities reacted late and with spotty enforcement of lockdowns appear to have been spared. Cambodia and Laos both had brief spates of infections when few social distancing measures were in place, but neither has recorded a new case in about three weeks.
Finally, most specialists agree that there may be no single reason for some countries to be hit and others missed. The answer is likely to be some combination of the above factors as well as one other mentioned by researchers: sheer luck.