Blood with a high concentration of plague bacilli is carried in the flea’s mouth and introduced directly to the bloodstream of the next person it bites. A rash develops within hours and death follows, almost invariably, within a period of one to three days, before buboes have had time to form.

Modern Epidemiology

The plague of 1665 was probably caused by a combination of population movement and seasonal changes in flea and rat ecology. London had suffered epidemics on a regular cycle: in 1563, 1593, 1603, and 1625. In the intervening years a steady number of people had died of the disease. A decline after 1649 indicates the development of collective immunity that might have led to plague’s demise but for the large-scale migration to London that accompanied the Restoration of 1660 and created a virgin population for the disease to attack.

Why the plague abated in 1665 so far as virtually to disappear from the British Isles is harder to explain. Plague was always most virulent in London, and the South East has always shown the highest concentration of people with blood group ‘A’, whom some studies show are more susceptible to flea bites. Where the most vulnerable part of a population experiences prolonged exposure to a disease, the strong develop immunities while the weak simply die. It is possible that Yersinia pestis mutated back towards its milder parent form, Yersinia pseudotuberculosis, initiating a stable, non-lethal relationship with the human host and causing a disease similar to typhoid that in turn gave partial immunity to plague. Human beings need not be the answer: it may have been rats that acquired immunity.

There is no substance to the theory that the Great Fire of 1666 caused plague to abate because brick and tiles replaced timber and thatch as building materials; the Great Fire caused little damage in the west of the city, where the plague had been most virulent. Nor is it true that the brown rat saved the day. Although it did have the advantage over the black rat of living underground and carrying fleas less partial to human blood, it did not arrive in large numbers until the 1730s. It is also very doubtful whether public and private hygiene improved sufficiently to eliminate the risk of infection.

Defoe’s Epidemiology

Defoe’s debt to contemporary epidemiology is charted in the Explanatory Notes to this edition but some general observations will establish the validity of his opinions, all with the caveat that his aim was to make a drama out of treatises. The three types of plague all appear to be present in the right proportions and Defoe’s observations about its treatment are largely accurate. The bubonic form predominates but Defoe also cites people who, ignorant of their infection, collapsed and died suddenly, suggesting the septicaemic variety. H.F.’s references to the ‘fatal Breath’ of victims is a legitimate construction of pneumonic plague which places him closer to modern science than many of his contemporaries, while his descriptions of sufferers’ mental disorder reflect plague’s tendency to attack the nervous system. Ilse Vickers has demonstrated Defoe’s interest in new scientific methodologies (Defoe and the New Sciences (Cambridge, 1996)) and it is fair to say that his enthusiasm is apparent in A Journal of the Plague Year.

That does not make its attempt to resolve old debates about the spread of plague accurate. Proponents of contagion argued that human contact was the key, while those who believed in miasma thought a poisonous cloud settled periodically, encouraged by hot weather, poor hygiene and—for some—gaseous effusions from the Earth’s core. Breathe in the cloud and you caught the disease. The miasma theory was popular because it appeared to explain how someone might catch the disease without coming into contact with anyone else, and because it sat more easily with descriptions of God’s judgement in the Old Testament. It depended on an idea H.F. emphatically rejects and modern science has proved true: that there are ‘invisible Creatures, who enter into the Body with the Breath’ (p. 65). But if H.F. is a contagionist he stretches his case to allow for a local or personal miasma: ‘no one in this whole Nation ever receiv’d the Sickness or Infection, but who receiv’d it in the ordinary Way of Infection from some Body, or the Cloaths, or touch, or stench of some Body that was infected before’ (p. 167). The idea may have arisen from medical observation but it also owes something to biblical descriptions of leprosy, referred to in the Journal (p. 211).

In a number of particulars Defoe was correct. Plague could be carried in clothes since fleas lodge in them. Bemused by prophylactics, H.F. still has his own ‘Preparation of strong Scent’ and fumigates his house, both of which might keep fleas and rats at bay.