Many books and articles have been written over the years about the mysterious illness which made Darwin into a semi-invalid after his return to Britain following the Beagle voyage. Everyone seems to have their own hypotheses. My own view, for what it's worth, is that we will never know the cause of Darwin's illness—and, indeed, that there may have been more than one cause (i.e. more than one illness).
In this short booklet, Robert Youngson, a retired opthalmic surgeon and physician, examines the various hypotheses put forward for Darwin's illness, explaining why none of them quite passes muster. I am not qualified to comment on his medical analyses, but they sound quite reasonable to me. The popular hypothesis that Darwin sufferred from Chaghas' Disease, for example, is problematic, Youngson claims, in that the disease is usually acquired in infancy, and has a very long gestation period.
Youngson is very balanced in his analyses, dismissing few of the contending hypotheses out of hand, merely pointing out the weaknesses with each one. He concludes the booklet by suggesting a hypothesis of his own (which the publisher has asked me not to reveal)—although he hedges his bets slightly by claiming that depression is another leading contender.
The booklet does contain one important error. Youngson claims: There are no indications that Darwin suffered any significant illnesses during the [Beagle] voyage. This is simply not the case. Darwin was confined to his bed from 20th September to the end of October, 1834. Darwin wrote to his sister from his sick-bed, describing this illness as follows:
I have been unwell & in bed for the last fortnight, & am now only able to sit up for a short time. As I want occupation I will try & fill this letter.— Returning from my excursion into the country I staid a few days at some Goldmines & whilst there I drank some Chichi a very weak, sour new made wine, this half poisoned me, I staid till I thought I was well; but my first days ride, which was a long one again disordered my stomach, & afterwards I could not get well; I quite lost my appetite & became very weak. I had a long distance to travel & I suffered very much; at last I arrived here quite exhausted. But Bynoe with a good deal of Calomel & rest has nearly put me right again & I am now only a little feeble.
Previous authors have suggested that this bout of illness might have been the result of an initial infection with Trypanosoma cruzi, the microorganism which causes Chagas' Disease.
But, as I said, I suspect we will never know for sure what caused Darwin's mysterious illness.
Disclosure: I received a free review copy of this book from the publishers.