(Image credit: bea_easter12_)
There are two kinds of people in this world. One is the kind of people who read quote marks as a sign of sarcasm, and the other is made up of people who use quote marks for emphasis. A lot. The result is a world full of signs designed or drawn by the second group that make the first group read the quoted words as facetious, and they have to laugh. The very words that the sign writer wanted to emphasize most are read as not being true at all. To the first group, the sign above reads as if they know they're never going to have an infection free hospital, so maybe we'll just pretend that's the goal. Then you laugh again when you see the entire graphic is one of hospital employees touching each other's hands, which is the fastest way possible to spread germs short of sneezing in someone's face. And that's just the beginning. The sheriff's department in Orange County, Florida, put the slogan Making a Difference on their patrol cars and not only put quote marks around it, but also rendered it in the Comic Sans font. How can anyone take that seriously?
(Image credit: Warm-Branch)
The reason we started reading quote marks as sarcasm is because journalists and authors would put quotes around phrases or even single words that other people said in order to distance themselves from those words, because the veracity of those words was in dispute. It's the same effect as putting "so-called" in front of a noun. Sarcasm quotes are useful in everyday writing, and even spilled over to the spoken word with the use of air quotes. But not everyone is up on that. See 50 ways quote marks have been used to comedic ends at Bored Panda.