how is a disease first created, is it created by someone’s cells like how cancer spreads or something else or does

Question: how is a disease first created, is it created by someone's cells like how cancer spreads or something else or does no-one know?
- Keywords:
- Royal Society of Chemistry: Find out more on:
  
  disease
  
  human
  
  microorganism
Asked by jodlington to Kieren, Rowena, Roy on 20 Mar 2014.
- Rowena Fletcher-Wood answered on 20 Mar 2014:
  
  You can get mutations in the body which cause diseases like Downs Syndrome, seemingly out of nowhere. This is when the body makes a “mistake” in copying DNA. Mutuations are often advantageous because they lead to new properties which can help us survive: they can also lead to diseases.
  
  You can also get diseases created by bacteria and viruses which change and adapt between their generations, being a little bit different from their parents like we are. This means a harmless virus or bacteria can eventually become a harmful one and replicate, or one kind of harmful one can evolve into another kind of harmless one.

Question: how is a disease first created, is it created by someone's cells like how cancer spreads or something else or does no-one know?