It is a terrible thing to see a disaster in the making and be unable to prevent it. Yet this is the prospect confronting us if we dare to look beyond the walls and parapets of a Britain besieged by the coronavirus invader. Tens of millions of people in poorer, less developed countries across the world face a looming catastrophe that appears as unstoppable as it is potentially lethal.

The moment has not quite arrived. But an axe is poised to fall on untold numbers of largely defenceless heads, a massacre almost too appalling to contemplate. As the relatively wealthy countries of the northern hemisphere engage in a noisy struggle to repulse Covid-19, alarm bells are ringing from south Asia to the Middle East and Africa. Mostly they have not yet been heard.

Anybody who has experienced the densely populated slums of Mumbai, Dhaka or Port-au-Prince knows how impossible are the concepts of social distancing and self-isolation for many of their inhabitants. Those who have seen the conditions in refugee camps in and around Syria or Somalia know how limited are the health facilities there, even at the best of times.

The World Health Organization has reported that at least half of the world’s population did not have access to essential medical care even before the pandemic struck. As the global equality charity Plan International Australia has ominously warned, many in these communities, particularly children, suffer from serious underlying conditions such as malaria, tuberculosis and malnutrition.

People living in conflict zones are doubly vulnerable. If you are an inhabitant of Idlib, for example, there is a good chance that any attempt to follow advice, stay at home and stay safe will be disrupted by a Russian or Syrian regime bombing raid. That’s one reason why António Guterres, the UN secretary-general, has called for a global ceasefire in the face of “our common enemy”. There is scant evidence that his plea is being heeded.

The UN has also warned that people in countries whose wellbeing has been damaged by unilateral sanctions, such as Venezuela, Zimbabwe and Cuba, could suffer disproportionately from the spread of coronavirus. This is already the case in Iran. All such sanctions should be lifted immediately, said the UN’s Hilal Elver, to help avoid virus-related food shortages. If the Trump administration heard her call, it has done little about it.

Food insecurity is a growing worry in sub-Saharan Africa, notably the Sahel region. If the disease takes hold there, the World Food Programme says, vital humanitarian supply chains in Burkina Faso, Mali and Niger could be further disrupted at a time when millions already face summer food shortages caused by drought and jihadist violence.

From war-scarred Afghanistan to the impoverished Central African Republic, where the Norwegian Refugee Council says there are three ventilators for five million people, fear of imminent disaster is compounded by a chronic lack of resources. And any responses to Covid-19 will be further hampered by unresolved political and security issues left to fester for years.

Gaza is a case in point. Two million Palestinians are crammed into an area of 365 sq km. According to the International Crisis Group, cases of coronavirus have been identified. Now Gaza is preparing for the worst. “A major outbreak would swiftly overwhelm Gaza’s healthcare system, which has been devastated by years of war and Israeli blockade. The death toll could be horrific,” the ICG said last week.

The potential economic impact on the developing world is also a matter of huge concern. Low- and middle-income countries have already been hurt by falling export demand, lower oil prices and a collapse in tourism. Nearly 80 governments have asked for emergency IMF funding, with one estimate suggesting a total bailout of at least £2tn may be necessary.

The resilience of vulnerable states suffering anticipated shocks of such unprecedented social and economic severity is greatly limited when compared, for example, with wealthier nations such as Britain that can call on unlimited borrowing. Lasting, possibly permanent pandemic-related damage increases the risk of social disorder and state violence, as seen in South Africa.

The incidence of Covid-19 in large swaths of Africa, south Asia and Latin America remains limited for now. But there is every reason to believe a virus that has ravaged and humbled the world’s two biggest economies - the US and China - will eventually spread infection to all corners of the globe.

It is not entirely true to say there is nothing we can do but watch. Governments and individuals should support the UN and its agencies; boost foreign aid budgets; donate to international charities; share expertise; offer refuge and succour; and, wherever possible, provide vital medical equipment – all this and more can and must be done. Covid-19 is a universal nightmare, a truly global crisis. All its victims without exception deserve help, wherever they live. If efforts to control the virus in the developing world fail, the human toll could be indescribable. And the disease could become endemic, repeatedly rebounding and recycling through migration, human contacts and second- or third-wave pandemics. It does not bear thinking about.