Chula ambulances were vehicles used by the Chula to heal their warriors on the battlefield and augment their strength for combat. They were filled with nanogenes and, like all Chula vehicles, could travel in time.
Jack Harkness stole an ambulance and a Chula warship. He used the ambulance to lure the TARDIS, which he thought contained Time Agents, and landed it in 1941 London. He wanted to con the "time agents" into thinking it was a Chula warship, and made sure a German bomb was to land on it, obliterating it. (TV: The Empty Child) Little did Jack realise that if this happened, the nanogenes would escape and cause the Empty Child plague.