How to get rid of odours in home that won't go away

[lightskyline]

Hi everyone,
A family member was ill and had neglected their flat and cleaning it, when clearing the flat I noticed a strong smell (not sure what it is) but it makes me sneeze/gives me a sore throat. I've had the flat professionally cleaned, repainted, new carpets but the smell still lingers and it still effects my throat whenever I'm in the flat for more than a few hours. Want it gone and not sure how to get rid of it, anyone have any tips? I'd really appreciate it xx

Have you been able to open all the windows regularly?

Is it possible the smell is coming from another flat?

I think it is definitely from this flat, well I've let out the flat the last year, I thought it would of gone and just lingered before because of the flat being empty but it still is here and I have asthma so just worried and want rid of it, going to open windows next few days but think I need to do something else too