While Covid-19 has now spread to almost all countries of the globe, some have been affected more than others, leading many to wonder how certain countries have been reporting hundreds of deaths — sometimes nearing 1,000 — in a day despite having decent healthcare systems.

As of today, April 16 2:30pm PST, US has the highest number of deaths in the world at 30,985, followed by Italy at 21,645 and Spain at 19,130.

China's death toll, since the first case reported in December, stands at 3,346.

Experts say that the massive spike in some parts of the world could be because of a combination of lack of testing, demographics and slow response.

Underestimating the crisis

One common reason for the virus turning into an unmanageable crisis is initial underestimations. Be it Iran, US, UK, Spain or Italy, all these nations initially believed the coronavirus will not hit them as hard as it did.

Rather than moving quickly to contain the spread, Iranian authorities, for example, continued to misreport numbers, downplay the situation and religious activities continued despite government appeals until the situation was completely out of hand.

Governments in Europe, however, failed to learn from Iran’s example and soon found themselves in even worse conditions.

“There was a lot of spread before people realised (the virus was present),” Alexander Edwards, an immunology expert at the University of Reading, told CNBC about the situation in Italy.

The United Kingdom chose to trod the herd immunity path until it was too late and Prime Minister Boris Johnson was forced to track back and close the country down.

The impact of not moving early and imposing a lockdown can be truly captured comparing UK’s response to its neighbour Ireland where things are much better, although far from being under control.

While Ireland cancelled its St Patrick’s Day parades scheduled for mid-March, concerts continued to be held in UK, The Guardian reported.

Similar reasons are being cited for varying trajectories of transmission and deaths in New York and California, with the former’s late response being held responsible for the appalling numbers despite reporting its first cases and deaths much later than California, according to Vox.

California’s experience likely reflects, at least in part, the value of quick, more proactive action, says the article.

We “need to shift to a proactive mentality rather than reactive”, Krutika Kuppalli, an infectious diseases physician and emerging leader in biosecurity fellow at Johns Hopkins Center for Health Security, was quoted as saying by Vox.

Lack of testing

Another difference between UK and Ireland is that the latter is testing for coronavirus at least twice the rate of the former, The Guardian reported.

Michael Tildesley, an epidemiologist at Warwick University, told CNBC that the death rate was linked to how many tests were being carried out as more testing allows for authorities to respond better to the situation. This is because many of those who contract the virus or are carriers of it do not show any symptoms.

Al Jazeera reported that Spanish authorities also ran into problems with testing as thousands of initial tests imported from China were found to be of limited reliability.

A senior Spanish official within the ministry of foreign affairs insisted that the government’s response has not been slower than other European countries.

“We have a simple message to all countries — test, test, test,” WHO Director General Tedros Adhanom Ghebreyesus had said last month in Geneva.

Without testing, cases could not be isolated and the chain of infection would not be broken, he added.

As with other countries, testing has proved to be of immense help in identifying hot spots in Pakistan. More tests, however, are required for the authorities to contain the virus from spreading.

Pakistan only recently carried out 5,540 coronavirus tests in a day — the highest number of tests conducted over a 24-hour period in the country so far. On average, Pakistan conducts 2,500 tests in a day.

Read: Is TB vaccine the reason behind Covid-19's less deadly effect in Pakistan? Experts are finding out

“If there’s enough testing around and people are willing to be tested, the brushfires can be identified and put out before the wildfire,” Martin, of UCSF, said while talking to Vox. “The only way that a society can function is if the brushfires are identified and put out.”

That doesn’t necessarily mean testing everyone, even those without symptoms, points out the article, adding: "That’s largely impractical; for one, people who test negative would have to be retested regularly to make sure they remain negative. But it does mean testing everyone with symptoms and the people they’ve come into contact with on top of getting them to isolate and quarantine."

Demographics

The elderly in both Spain and Italy, as in many other countries of the world, were among the first to be affected by the virus, disproportionately represented in deaths because of the virus.

Italy, according to OECD data, has the second highest percentage of elderly — 65 years and above — only behind Japan. In addition, high levels of contact between the young and the old.

"Elderly Italian people, while most of them live by themselves, are not isolated, and their life is characterised by a much more intense interaction with their children and younger population compared to other countries," Linda Laura Sabbadini, central director of the Italian National Institute of Statistics, told Al Jazeera.

"We have many elderly people with numerous illnesses who were able to live longer thanks to extensive care, but these people were more fragile than others," Massimo Galli, head of the infectious disease unit at Milan’s Sacco Hospital, said adding that many patients who died due to coronavirus were already suffering from other serious diseases.

More: 'It's the math, stupid': Why we may be reading the Covid-19 numbers all wrong

While Spain does not have an elderly population at the same proportions, it is similar in its culture of greater interaction between the young and the old.

Population density is another factor that is though of as contributing to the problem. “There's a huge density of people in cities like Madrid or on the Mediterranean coast in particular, and a lot of blocks of flats in cities' outlying suburbs,” Alberto Mataran, a professor of Environmental Sciences at the University of Granada told Al Jazeera.

New York, the worst-hit US city, is also the densest in the country by a margin, according to US census data.

Possible factors include better containment measures, or just the randomness of who happened to contract the virus first, and where they went, noted a New York Times article, adding: "Still, public health experts said that density was likely the biggest reason for why the virus has torn through New York City and not yet hit to the same degree elsewhere."

“Density is really an enemy in a situation like this,” Dr Steven Goodman, an epidemiologist at Stanford University told NYT.

“With large population centres, where people are interacting with more people all the time, that’s where it’s going to spread the fastest.”

According to the article, researchers have noticed that New York City has a similar population and a somewhat similar density to that of Wuhan, the Chinese city where the virus originated.