The nature of the plague, as an occurrence beyond all accounting, not only in other respects affected each person more harshly than is humanly bearable but also showed itself in the following way above all to be something completely different from the familiar diseases: all the birds and animals that feed on man either did not approach, even though there were many unburied, or died if they tasted them. And as proof: the absence of birds of this type was unmistakable, and they were seen neither elsewhere nor in this context; dogs, on the other hand, made it more possible to notice the results because they live with men.
"All the birds": the argument is not quite clear; apparently Thucydides means the corpses were toxic enough to kill scavengers, which could be more directly observed of dogs, and inferred of birds by their absence - in which case it is strange that none were seen "in this context," i.e., eating the dead.
 The plague, then - to omit many other peculiarities in the way it happened to occur somewhat differently for one person compared with another - was like this in overall character. And during this time none of the usual diseases troubled them, or even if any did it ended in this. Some died in neglect, others, when they had been given a great deal of attention. And no single cure was established, practically speaking, whose application could bring relief; for what had helped one person actually harmed someone else.
"No single cure": It is remarkable that no single cure is described here.
Thucydides The Pelopponesian War Book Two, trans S Lattimore (1998), p99
For myself, I am not quite clear why the argument in  is not quite clear or what is strange about a precise claim that Thucydides does not seem anyway to be making; nor why it is remarkable that not one cure is described here when all have variously failed.
But that is one of the joys of a really first-rate edition [insert plug for Hackett Classics ed]: footnotes at the foot of the page which demand footnotes.