Daud makes mention of an ability that makes him virtually immune to poisons and toxins. He's referring to an upgraded version of your own Vitality power. If Corvo had been Marked for longer, maybe he would have been able to resist even a full dose of the poison he is given late in the game. Likewise, Granny Rags claims at one point, "Don't toy with me! I may be blind, but I have the sight!" She is of course referring to the dark vision power. If you're fast enough, you can see that Corvo is closing his eyes whenever he uses Dark Vision; you need to be blind before you can use it.
According to the Heart, It seems like everyone in Dunwall is a narcissist of some sort. There are several genuinely decent people, but those are the exception rather than the rule. This looks like a straight example of Humans Are Bastards, but consider the world these people are living in. Everything from the hounds to the barnacles are incredibly dangerous, and even the common housefly is stinging and parasitic. Furthermore,all the examples you see in the game are just the Urban wildlife; you don't even glimpse the truly wild places, although the severe lethality of the creatures inhabiting those areas are hinted at throughout the game. In a world where Everything Is Trying to Kill You, a greater emphasis on your personal survival is far more understandable. Historians have speculated that empathy and selflessness only became virtues after humanity became secure at the top of the food chain, and Dunwall has yet to achieve that in spite of the technology at their disposal.
The conflict between the feral rats and the humans of Dunwall is eerily similar to the conflict between the humans and the whales; a greater quantity of smaller yet well-organized mammals are subduing much larger prey through coordination and numbers. I’d imagine that a human being run down by a pack of rats would feel quite similar to a whale being caught by a whaling ship.
The title Dishonored takes a whole new meaning if you are familiar with the Honor Culture of the 1700s and 1800s. Honor was not being ethically virtuous, so much as being worthy; a man could be extremely amoral and completely honorable at the same time. (Duels were not honorable because you were attacking an offender, but because you were exposing yourself to danger to prove your integrity. Refusing to shoot at your opponent would be a terrible insult to them.) Even in a pacifist playthrough, Corvo deploys stealth, uses magic and technology his enemies lack, and disgraces his marks while they are helpless/unaware. Although Corvo's methods resemble a southern European vendetta in style and execution, by the standards of English honor code he is truly Dishonored. This is even more brilliant if you follow the in-game lore. Corvo is from Serkonos (based on southern Europe) while Dunwall is part of Gristol (based on England). Historically,there was often friction between northern Europeans and southern Europeans because their definition of "honor" differed. In the north, it meant upholding your integrity and worth in a composed, dignified, and public manner. In the south, it meant respecting your family by avenging every insult to them through any means necessary.
It may seem odd that all the Lord Regent’s key supporters are, without exception, unsympathetic sociopaths. Since his duplicity isn’t publicly known, it wouldn’t be implausible for honest, upright individuals to support his rule while ignorant of his crimes. However, taking a closer look at the Lord Regent’s personality reveals that he is dismissive of intangible, unquantifiable traits such as “honesty” and “trust”. The Lord Regent appoints such amoral people in the first place because they have an obvious, tangible interest in remaining loyal, mainly because of the “favors” he can grant them as the ruler of Dunwall; he doesn't trust people who have no personal (corrupt) reason for following him.
Delilah's enmity towards Daud in the Knife of Dunwall DLC may be because his immunity to possession makes him a threat to her Grand Theft Me plan and powers, especially as he could conceivably pass that ability to Emily.
Throughout the game, Corvo is exposed to a ridiculous amount of plague germs - more so the more chaotic he is. He's fighting weepers, he's swimming in the river, he walks around in the sewers, he runs around with open wounds, et cetera. Even if he does down Elixirs like they're candy (and in a perfect Ghost run, he shouldn't need them), you'd think by the end of the game he'd be weeping himself... until you poke around in some books and find this poem. It describes a Serkonan woman dying from the Rat Plague, or some version of it. Given that the poem was written in Old Serkonan, this means that some centuries back, the Rat Plague came to Serkonos and wiped out a portion of the population. Here's the important part: everyone who was left was then immune to the Plague, and passed that immunity on to their kids. Corvo is Serkonan. It's very likely that of all the people in Dunwall, he's one of the very few who is genetically immune to the Rat Plague. Same would go for other Serkonans in the game, including Daud and Emily.
Or he's immune because of his supernatural abilities.
Many would wonder why the Outsider would grant Corvo the powers he did, especially given the fact that, with the "Mostly Blood And Steel" achievement, it would show that it is possible for Corvo to complete most, if not all, of his missions without those powers. However, that is the point: the only easy way Corvo can complete his missions without powers is to turn Dunwall into an abattoir... something he is fully capable of doing. He'd have to slaughter his way through each mission; it would be very hard to get through a nonlethal playthrough that way. And that's why the Outsider gave Corvo those powers: to see if Corvo would, when another option was feasible, choose to take the high road, and instead of being a bloody, ruthless murderer, he would choose to be the better man.
Why are Whalers so vulnerable to possession, despite being well versed in the Outsider's powers? Easy, it's not a power that Daud has access to, so obviously his followers get blindsided by it.
Possessing someone causes them to vomit once you leave them. As mentioned in Game Breaker, possessing Whalers and leading them to a secluded location to knock them out is one of the easiest ways to get through the Flooded District. Whalers wear gas masks. If you use that strategy, you're going to cause a lot of Whalers to fill up their own masks with vomit. Plus, they use those masks to breathe in the plague infested air. You just made it harder for them to avoid becoming weepers. Assuming, of course, that they don't drown in their own vomit-filled masks.
Even in a no-kill playthrough, many of your actions have the potential to ruin people's lives. Disable a wall of light in an infested area? The guards there may be overrun by weepers the next day. Loot a low-rent apartment? You just deprived a poor, starving woman of her only remaining funds. Knock out a guard and dump him in a corner? He could be sacked or redeployed to the flooded district for napping on the job, being unable to prove that he was attacked. One particular example is in Dr. Galvanni's house; you find a note by the Dr. which states he wont have to feed his lab rats for a few days because of an "altercation with the maid". (There is a severed arm on a platter in the same room.) You find another note nearby where he warns the current maid to avoid his lab at all costs, or suffer dire consequences. Tamper with the lab and avoid detection, and the good Dr. will be all too likely to blame the poor girl... This is shown to be the case in the first mission of The Brigmore Witches DLC. A number of guards who were on duty while Corvo escaped from Coldridge are being executed for either negligence or on the off-chance that they actually assisted him.