Thename is misleading. The commonest natural hosts of monkeypox are the small rodents that live in rainforests in West and Central Africa. But it can infect a very wide range of animal species, including humans. Unlike bat coronaviruses it doesn’t have to mutate to move from one species to another. When smallpox was common, monkeypox cases went unrecognised, because the two diseases are so similar they couldn’t be distinguished in the field. Without a definitive laboratory test, it wasn’t possible to answer the two fundamentally important questions facing the smallpox eradicators at the end of their project: had smallpox actually gone (were the remaining cases in fact monkeypox), and did it have an animal reservoir (as monkeypox did)?

The World Health Organisation set up committees and handed out grants. Various laboratories went to work. One of them was in Birmingham. In 1977 I visited it to assess its progress in developing a new test to distinguish smallpox from monkeypox using virus proteins labelled with radioactive amino acids. It produced excellent results. But on 11 August 1978, Janet Parker, a photographer who worked upstairs from the pox lab, developed a flu-like illness, and then a vesicular rash. On 24 August electron microscopy of the vesicle’s contents showed brick-shaped virus particles. She had smallpox. The vaccination of contacts began. On 31 August the media called the head of department’s home forty times. The next morning, he cut his own throat (he died six days later). Janet Parker died on 11 September, the last person in the world to be killed by smallpox – a laboratory escape, and one in which monkeypox played a part.

The WHO finally declared the successful eradication of smallpox in 1980. Today, monkeypox is commonest in the Democratic Republic of the Congo. Outbreaks there are associated with hunting, skinning and eating infected rodents; some children have died from it. Human-to-human spread has occurred, but the chains of transmission aren’t long because of the low probability of catching an infection by casual contact. A genetically distinct and less virulent monkeypox variant occurs in West African rodents. It causes a non-lethal illness in humans. After a day or two of fever, prostration, and the development of swollen lymph glands, a rash starts to appear, usually on peripheral parts of the body, sometimes on genitalia and the tongue. The spots dry up after a few weeks.

On 9 April 2003 a wholesale pet shop in the American Midwest imported six species of rodents from Ghana. They were housed with two hundred prairie dogs, which were then sold as pets. The prairie dog keepers began to fall ill with monkeypox in May; 72 people were infected, all recovered, and none spread the infection to their human contacts. In Europe there have been only a few monkeypox cases in recent years, all involving a history of travel to Africa. So the big outbreak that started in the middle of May – affecting individuals in North America, Australia and many European countries, but not apparently involving travel to Africa – took everyone by surprise. Was this a new variant of concern?

Almost certainly not. Pox viruses are DNA viruses. They evolve much more slowly than RNA viruses such as influenza and Sars-CoV-2. The explanation for this outbreak of monkeypox probably lies in human behaviour and its effect on the transmission of infection. The cases in the outbreak have predominantly been in men who have sex with men (MSM). Monkeypox has joined microbes like hepatitis A and Shigella flexneri in showing a capacity for sexual transmission. In the past Shigella wasn’t an STD. My grandfather became an asylum attendant (nurse in a psychiatric hospital) in 1893. Two years later Shigella struck in a big way, killing many patients. For the next thirty years it was commoner in psychiatric hospitals than in the community, causing ‘asylum dysentery’. Closing the asylums resolved the problem (although many people with dementia are now in care homes, where they have suffered high death rates from Sars-CoV-2 instead). Today the most reliable way to catch Shigella is to go on a package holiday to the tropics; there are occasional outbreaks on cruise liners.

Shigella is transmitted by the faecal-oral route, and its transmission between MSM by direct or indirect oral-anal contact explains its sexual transmission. Outbreaks of hepatitis A and Shigella among MSM in recent years have been in men with multiple sexual partners. Hepatitis A infections in the UK have been caused by viruses related to strains found in South America, and elsewhere in Europe by viruses related to strains in Japan and Taiwan. Contact tracers will doubtless be investigating the degree to which similar risk factors apply to the current outbreak, and whether monkeypox has become an STD because of direct sexual transmission from vesicles on tongues or genitals, or by intimate person-to-person contact facilitating the direct inoculation of the skin from the virus-laden rash or the inhalation of dried crusts from a healing rash.

The good news is that genome sequencing has shown that the current monkeypox virus belongs to the less virulent strain from West Africa; that smallpox vaccine gives good protection, even when administered after exposure; and that the staff in STD clinics have more experience as contact tracers than any other workers in public health. Most important, monkeypox is about as different from HIV and Sars-CoV-2 as could be: it doesn’t set up long-term potentially lethal chronic infections, it doesn’t mutate rapidly, and the worst residue of the acute illness it does cause – which resolves spontaneously – is the facial scarring caused by its pocks, of the kind that affected Lady Mary Wortley Montagu after her recovery from smallpox in 1715.

The likelihood of finding how the virus got out of Africa is low, and it is too early to tell whether the current outbreak will fizzle out or linger in the MSM network. It is a manifestation of how we live now. ‘Infectious disease is one of the few genuine adventures left in the world,’ the microbiologist Hans Zinsser wrote in his classic Rats, Lice and History (1935):

The dragons are all dead, and the lance grows rusty in the chimney corner … About the only genuine sporting proposition … is the war against those ferocious little fellow creatures, which lurk in the dark corners and stalk us in the bodies of rats, mice and all kinds of domestic animals … and waylay us in our food and drink and even in our love.

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