>>> i heard they can cause legionnaires disease
B0LL0X!
There was a scare story in the US about an outbreak of Legionnaires' Disease that appeared to have been due to the use of an 'evaporative cooler' (similar to the fan you refer to). However it was later traced to a nearby cooling tower.
A study by Puckorius, Thomas and Augspurger, entitled 'Why Evaporative Coolers Have Not Caused Legionnaires’ Disease', which appeared in the journal of the American Society of Heating, Refrigerating and Air-Conditioning Engineers in January 1995 should have put paid to the unfounded rumours that there's a health risk to using evaporative cooling devices but, clearly, it's failed to do so!
Mark Reddick, of the Sleepdvisor website, has suggested some reason why sleeping with a fan on might not be a good idea. (His advice was picked up by the national press and appeared in The Sun, The Mirror, The Metro and elsewhere). However even he doesn't mention Legionnaires' Disease (probably for the very good reason that there's no link whatsoever):
https://www.sleepadvisor.org/sleeping-with-a-fan-on/