How do you know how someone has got a disease???

Question: How do you know how someone has got a disease???
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Asked by 974merf22 to Waqar on 17 Jun 2016.
- Waqar Ahmed answered on 17 Jun 2016:
  
  Do you mean in the research I do?
  
  When we collect breath, using technology we are able to measure what chemicals there are, and how much of them there are.
  Most diseases like cancer and infections, especially from the lungs, can stress organs and make them work harder or slow them down. Once this happpens, the chemicals released by these organs change. We can measure the chemical changes in breath, blood and urine. This means we can know when someone has a disease, even before the doctor knows.
  
  This is because the doctor relies on looking at symptoms like a fever or a headache – which comes waaaaaay after any chemical
  changes.

Question: How do you know how someone has got a disease???