It seems that people use the word machalah to refer to cancer not only as a translation of the word (although there might be a more specific and correct translation) but because they do not want to say that someone has cancer. Why do people do this?
The Gemara refers to Tzaraas as "Davar Acher" - "lit. Something else".
Satmar rov reb yoel said if you call it that then it will hopefully dissapear