Some further considerations: it would be fair to object that it is war, and not disease, that is the province of Mars. This is true--and it's also true that the plague of 430 BCE coincided with a war between Athens and Sparta. Mars was Sparta's patron god, for obvious reasons, and Lucretius could have ended more explicitly with war as Santayana proposes. But disease works better for Lucretius on a moral level. Philosophy was for him a kind of medicine, and it was a medicine that people needed even if they didn't know it. Diogenes of Oenoanda makes it very explicit;
QuoteBut, as I have said before, the majority of people suffer from a common disease, as in a plague, with their false notions about things, and their number is increasing (for in mutual emulation they catch the disease from one another, like sheep)
And John Stuart Mill says this of his father:
QuoteAs it was, his aversion to religion, in the sense usually attached to the term, was of the same kind with that of Lucretius: he regarded it with the feelings due not to a mere mental delusion, but to a great moral evil. He looked upon it as the greatest enemy of morality: first, by setting up fictitious excellences—belief in creeds, devotional feelings, and ceremonies, not connected with the good of human-kind—and causing these to be accepted as substitutes for genuine virtues: but above all, by radically vitiating the standard of morals; making it consist in doing the will of a being, on whom it lavishes indeed all the phrases of adulation, but whom in sober truth it depicts as eminently hateful.
A further objection might be in the use of the word fetus. 'Offspring' or 'child' would be a non-standard usage, but it is justified at least in Horace: Germania quos horrida parturit Fetus. This translates as far as I can tell to Germany gave birth to a horrible child.
Lastly, here is an article which I have not read but which reinforces the connection between the beginning and ending of the poem.