bugs.
my sister bought me a bunch of flowers many years ago - and they had a beetle larvae in them - it has taken me many years to get rid of the little buggers
i found a little pile of them right under where the flowers were on the mantle piece under the rug - i killed them all but some had obviously escaped
i researched and found they do come on on flowers.
at first they were everywhere, it was horrible - i went mad trying to kill every single one, spraying, squashing, hoovering etc, but i hardly ever see one now - but do still find the odd few now and again - drives me mad.
a proper infestation would cost a lot of money to properly treat in a hospital, as they are very hard to completely get rid off - they get into everything and are tiny when they are babies so easily missed ... then the whole cycle starts again, as they have a long gestation
... and they cant be spraying insecticide, or letting off smoke bomb things etc in a hospital.
and they cant just hoover up if an area is contaminated like you could in a normal house.
there may well be other bugs too...
i also think the water can be pretty icky too - if a vase got knocked over you have a big spill of manky water that needs properly cleaning up with antibac etc
i daresay they have reasonable explanation for doing it - im sure the nurses etc would want to have to deal with huffing relatives who don't understand all the time, if they didn't have to.
maybe the myth about them taking oxygen was started as a way to shut moaning relatives up, hehe.