It's hard to think about Infect without thinking about Tom The Boss Ross. Ross made it to the finals of Grand Prix New Jersey, which, at over 4000 players, was the largest Legacy event in history. Since then, Ross has continued to have tremendous success with the deck in multiple formats. It's clear that Ross understands the archetype better than anyone else in the world.
Infect continues the theme of fast and resilient combo strategies being successful in Legacy. With access to cards like Berserk and Become Immense, it doesn't take many cards for infect to kill the opponent. In Legacy, the deck gets to protect its combo with Force of Will.
The Legacy version of Infect is similar to its Modern counterpart, but this version has several clear upgrades because of a wider selection of cards to choose from in blue and green. While splashing would be easy here, there really aren't many cards in other colors the deck would want, certainly not in the main deck. I have found that in general there are enough strong sideboard cards in blue and green that the splash isn't that significant. As far as the creature base, it is identical to the modern version.
There are not that many Infect creatures that have been printed, and none of them are legal in Legacy and not Modern, so it makes sense that there would be no difference here across the two formats. There are 12 Infect threats officially, but there are ways of finding those threats, which make it seem like there are more. In Legacy, even the fair decks don't need to play that many creatures, since with all of the card filtration and draw those threats easier to find.
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