It was in April 1665, that the first recorded death relating to the Great Plague of London occurred. One month earlier, on March 24, 1665, Parliament had promulgated the Five Mile Act, which came on the heels of the 1664 Conventicle Act, and the 1662 Act of Uniformity, which led to the ejection and continued persecution of 2,000 non-conformist ministers in England. It is believed that this bubonic plague arrived in England from the Netherlands, which suffered the Plague in 1663-1664. The docks of London were hit first, and then the disease spread throughout that summer, through in fact September 1666, ending the lives of an estimated 100,000 citizens, about 20% of the population of London. It was in September 1666, that the Great Fire of London struck, heaping disaster upon disaster. The Fire destroyed St. Paul's Cathedral (later rebuilt by Sir Christopher Wren) and thousands of other structures, along with an unknown number of deaths, but helped bring an end to London's last major plague.
During this period of affliction, certain ministers who had previously been ejected from their pulpits, bearing in mind what the Apostle John taught -- "My little children, let us not love in word, neither in tongue; but in deed and in truth" (1 John 3.18) -- made the courageous decision to stay in London and minister to her suffering citizens, despite the risks to their own personal well-being.
Gerald Robertson Cragg, Puritanism in the Period of the Great Persecution, 1660-1688, pp. 13-14:
The Great Plague, which had compelled parliament to meet in Oxford, persuaded many of the conformist clergy to fly from London. The churches stood empty amid a population haunted by the fear of disease and death. This was a challenge which the ejected ministers felt they could not ignore. They took possession of the vacant pulpits, and the people listened meekly to their message of judgment and mercy. In the wake of the Plague came the Fire of London. The nonconformists it was apparent that they were living in exceptional times. When God had 'a controversy with his people' those who had heard his call could not be silent. It was therefore right that they should meet more publicly than heretofore. The increased congregations they attracted seemed to justify a bolder policy. With remarkable industry they began to erect new meeting houses in which to worship, but sometimes when the building was finished it was seized for the benefit of a parish which had lost its church but had shown no comparable zeal in replacing it.
The increased activity of the nonconformists inevitably attracted attention, but their devoted service during the recent troubles had temporarily checked the persecuting spirit.
Among the non-conformist clergy who made the decision to stay were, according to Richard Baxter and Edmund Calamy the Historian, James Janeway, Edward Chester, Edward Turner, John Grimes, and Robert Franklin. These men are all true heroes. I would take special note of one other minister, Thomas Vincent, who is known for his services to the afflicted, and who wrote a first-hand account of his ministry during both the Great Plague and the Great Fire, entitled, God's Terrible Voice in the City, in which he records how the Plague struck even inside his own household. Vincent and those who stayed with him and "looked death in the face," in order to bear witness to the gospel of Jesus Christ in midst of such suffering, are rightly remembered among the honor roll of noble Christian heroes of the faith.
Edmund Calamy the Historian, Continuation, pp. 31-33:
Upon the Progress of the Distemper in the City, he acquainted his good Friend with his Design to quit that Employment [as a teacher at Doolittle's Academy], and apply himself peculiarly to the Visitation of the Sick, and the instructing of the Sound, in that Time of pressing Necessity. Mr. D. endeavour'd to dissuade him, by representing the Danger he must run; that he thought he had no Call to it, being then otherwise employ'd; and that it was rather advisable he should reserve himself for farther Service to the rising Age, in that Station wherein he then was so usefully fix'd. Mr. Vincent not being satisfy'd to desist from his intended Service, they agreed to desire the Advice of their Brethren, in and about the City upon the Case. When Mr. D. had represented his reasons at large, Mr. Vincent acquainted his Brethren, that he had very seriously consider'd the Matter before he had come to a Resolution: He had carefully examin'd the State of his own Soul, and could look Death in the Face with Comfort: He found to Timerousness and Dread in his own Temper: He thought it was absolutely necessary that such vast Numbers of dying People should have some Spiritual Assistance: He could have no Prospect of Service in the Exercise of his Ministry through his whole Life like that which now offer'd itself: He had often committed the Case and himself to God in Prayer; and upon the whole had solemnly devoted himself to the Service of God and Souls upon this Occasion: And therefore hoped none of them would endeavour to weaken his Hands in this Work. When the Ministers present had heard him out, they unanimously declar'd their Satisfaction and Joy, that they apprehended the Matter was of God, and concurr'd in their Prayers for his Protection and Success.
He went out hereupon to his Work with the greatest Firmness and Assiduity. He constantly preach'd every Lord's Day through the whole Visitation, either at Aldgate-Church, or Great St. Hellen's in Bishopsgate-Street, or Allhallows in Thames-street, or some other Church. His Subjects were the most moving and important; and his Management of them most pathetick and searching. It was a general Inquiry through the preceding Week where he was to preach: Multitudes follow'd him wherever he went: And he preach'd not a Sermon by which there were not several awaken'd, and as far as Men could judge brought home to God. Besides this, he without the least Terror visited every one that sent for him, doing the best Offices he could for them in their last Extremities: Being instant in Season and out of Season to save Souls from death.
David Bogue & James Bennett, wrote of Thomas Vicent, specifically, in The History of the Dissenters, From the Revolution in 1688 to the Year 1808, Vol. 1, pp. 233:
The world has its heroes, whom it holds up to universal admiration in the page of history. Here the church of Christ presents to us one of hers. The world calls us particularly to admire them as they advance to some arduous enterprize, where perils and death stare them in the face, but advancing with tranquillity of mind, with firmness of step, and determined either to conquer or to die. But which of them can be compared to this man ! He sees the inhabitants of a city, from which he had been cast out as unworthy of the name of a minister of Christ, dying by the pestilence which was augmenting its destructive fury from day to day; and he cannot be restrained from rushing into the midst of them to rescue their immortal souls from miseries infinitely greater. He hastens into churches from which he was driven out, and proclaims to listening thousands the glad tidings of salvation, in pulpits, for entering which the law of the land dooms him to a dungeon; but a stronger law, the law of love to God and man, constrains him to publish the mercy of the Gospel to souls on the very brink of eternity. He goes into the house of pestilence, and the chambers of mortal disease, wherever the voice of misery invites him. His exhortations, his counsels, and his prayers, are ever at their call; and they ever flow from a compassionate heart, tenderly sympathising in their distress, and burning with zeal for their salvation. Great was the success of his labours; and during the plague a harvest of souls was reaped, exceeding what results from the painful exertions of many a faithful minister during the course of a long life of zeal....As long as Christ has on earth a Church, animated with zeal for the glory of his name, Thomas Vincent will live. His writings breathe with the most affectionate ardour for the salvation of immortal souls; they savour of the minister who, for months, preached to congregations infected with the plague.17
17 His example was followed by his non-conforming brethren, Messrs. Chester, Janeway, Turner, Grimes, Franklin, and some others. Drs. [Anthony] Walker, [Thomas] Horton, and Meriton, and a few others of the conforming clergy, remained at their posts, but the generality fled.
For more information on this calamity of calamities, and the characters who played out the drama of this tragedy, I recommend reading eyewitness accounts such as Thomas Vincent, God's Terrible Voice in the City, and Nathaniel Hodges' Loimologia, or, an historical Account of the Plague in London in 1665; as well as well-researched works of historical fiction such as Daniel Defoe's A Journal of the Plague-Year, and Mrs. Margaret Oliphant, Caleb Field: A Tale of the Puritans.