Even as the very thought of a contagion that might be spreading globally drives panic, border closures, and lockdowns, there has never been a systematic articulation of the scientific principles to determine a society’s calibrated response. Instead, we have an assortment of opinions, often informed by pseudo-scientific methods such as epidemiological models and ethics. 

The underlying idea of contagion is old enough. For instance, Ibn Sina wrote about the “transmission of disease from person to person” in his c.1025 book, “Canons of Medicine” and listed a number of examples. But much of this ancient thinking was incorrect. In 1800, Charles Maclean proved that many diseases once thought to be contagious were not so. He also found that quarantine (isolation of the healthy) in response to contagion causes great harm, regardless of the contagiousness of a disease. In this piece I propose a general theory of contagion to guide society’s isolation and quarantine response.

1. Only contagious diseases are fit for isolation

Most public health experts don’t seem to know that only contagious diseases are potentially suitable for isolation. As recently as on 16 April 2021, a major media outlet in Australia, ABC News, conducted a “fact check” in which it argued that covid lockdowns were modelled on plague “lockdowns”. It claimed that: “In Britain, King Charles II imposed a lockdown on people in towns, cities and ports during the great plague of the 1660s. He controlled the movement of villagers and banned public gatherings in his ‘Rules and Orders’ declaration”. The experts whom ABC consulted didn’t know that such restrictions couldn’t have done anything for the plague. For over a century now, we have known that plague is transmitted by fleas that live on rats. Isolation of plague patients, or quarantine of healthy people, cannot possibly stop rats and fleas from moving about. Similar, illogical and unsuccessful attempts were made in past to control non-contagious cholera and yellow fever. Cholera transmits through water and yellow fever via mosquitoes. The first principle, therefore, is that isolation or quarantine cannot stop non-contagious diseases.

2. Principles to determine whether a disease is suitable for isolation

Once we confirm that a disease is contagious, its prospects of containment can be informed by a decision tree with the following questions:

i. Does virus/bacterial excretion occur before symptoms?

ii. Are sick (symptomatic) persons typically mobile?

iii. Is the disease difficult to diagnose?

1. Is transmission mainly by aerosols?
2. If via droplets/ fluids, does the disease have a high R0?

A “yes” answer to any of these questions eliminates, or at least significantly truncates the prospect of isolation. For the fifth question, there is a qualifier, i.e. Is the high R0 due to high-volume excretion at a short distance? If so (this applies to pertussis), isolation might still be beneficial. A scoring system based on this method yields 91 points each for smallpox, Ebola and SARS, 63 points for tuberculosis, 36 and 32 points for pertussis and scarlet fever, and less than 25 points each for seasonal flu, covid and measles. A score of 90+ suggests that strong isolation could even eradicate the disease. A score of 61-90 indicates moderate benefits from isolation, such as through institutional isolation. A score of 31-60 suggests small but positive benefits from isolation by natural processes such as reduced over-crowding. Finally, a score that is less than 30 suggests that isolation can do nothing to reduce transmission.

Other characteristics of a disease may also need to be considered. If the virus or bacteria has relatively large in size, N95 masks can help. If hospitalisation is necessary, additional precautions are needed. If fomites are an issue, they need to be carefully managed. Finally, human factors play a crucial role in the success of isolation. As a general rule, most contagious diseases are hard, if not impossible to eradicate.

3. Strong isolation for smallpox, SARS, and ebola

Given its high isolation score, theory suggests that smallpox is potentially fit for eradication. And indeed, this possibility was proven as long ago as in 1784 by John Haygarth. In 1790, Dr Robert Walker proposed the elimination of smallpox by isolation alone, without inoculation. It was during the W.H.O.’s smallpox eradication program (SEP) that isolation was put to good use and as expected, it did eradicate the disease. Resistance since 1790 from the medical community to isolate smallpox only delayed its eradication. Likewise, SARS was eliminated, if not eradicated, through isolation. Ebola outbreaks have also been brought under control through this isolation.

4. Moderate and weak isolation for tuberculosis, pertussis

Both TB and Pertussis declined by over 90% before any antibiotics or vaccine. The isolation score of these diseases suggests that isolation is beneficial. Reduced over-crowding through better housing improved natural isolation of these diseases. Further, a sharp decline in fertility from the second half of the 19th century in England meant that there were fewer children in each household, hence the spread of pertussis reduced. Better nutrition – as people became wealthier – also helped. Finally, improved awareness of hygiene meant that the previously widespread practice of spitting (which was particularly problematic for TB) came to an end.  Institutionalised isolation for TB remained rather limited, with only a small proportion of the nearly quarter million TB patients in the UK being isolated in sanatoriums.

5. Quarantine is impossible even for smallpox

Quarantine (isolation of the healthy) is the obverse of isolation of the sick. It can only work if isolation of the sick can prevent transmission of disease. If isolation can’t work, quarantine will necessarily fail. But we know that quarantine can’t work even for smallpox. As Joshua S. Loomis pointed out in 2018, “quarantine measures failed to adequately contain smallpox for most of history”. Smallpox patients typically become immobile when their symptoms emerge, so they don’t travel. On the other hand, if someone who is incubating smallpox travels, he cannot transmit the disease – which only transmits after symptoms. In a 2002 paper, Thomas Mack found that not a single case of smallpox in Europe after World War II “involved disease contracted on an airplane, train, or bus”. Accordingly, as reported in 2006 by Donald Henderson, the SEP decided “never to impose quarantine, i.e. forcing contacts of patients who were otherwise well to be sequestered in their home or in a building”. Thus, when quarantine is worthless even for smallpox, the idea that it can contain any contagious disease is delusional.

END OF ARTICLE