When Danny left Amity he was pretty sure it would be a quick trip. He wasn't expecting a few days' visit to Gotham to check some annoying "co-workers" of Vlad's, a gala and a serious case of fever.
Being fair, Danny didn't know that ghosts could get sick, or how much damage this could do. His powers went completely haywire, activating and deactivating in the middle of the street, and it was only thanks to Vlad that he was able to come up with an excuse or two.
Vlad obviously didn't expect him to get sick either, if his disgusted face was anything to go by. The two had a truce of sorts but it was obvious they were only on friendly terms thanks to Jack Fenton.
Danny didn't feel like locking himself in a room and drinking soup. He wanted to call Frostbite, but he was too far away from the Realms, and the halfa refused to let Vlad check on him.
So he ended up on the streets of Gotham, spreading early winter and ice stalagmites in his wake. It was just luck that Vlad found him before the bats, who were obviously interested in the new development. Danny wondered how he was supposed to attend the gala in those conditions.