Sergeant Scott's 'illness' was an acute attack of PTSD due to all the murders he's investigated in Midsomer.
He resigned from the force and is now growing vegetable marrows in a remote village in Kent. Scott was murdered.
The time he calls in sick, he's actually suffering the effects of a poison which eventually kills him. The culprit turns out to be a brilliant psychopath that Barnaby had put away years earlier. Barnaby and Jones bring the killer to justice, but Barnaby is so shaken by Scott's death that he never mentions him again. An explanation for Midsommer.
Murders are ridiculously common in Midsommer. All the old fashioned little villages of Midsomer have in common that a) they all represent old fashioned countryside b) Midsommer has a ridiculously high murder rate exceeding most metropolitan cities. The medical examiner thought a death by natural causes to be such a rare event as to be worthy of joking about it. c) Every time someone tries to modernize anything, it usually results in a new string of murders and at least one of the modernizers ends up dead d) despite all the murders everyone seems rather calm about the whole thing, even Barnaby whose job is to catch killers usually just lets suspects walk around freely without making any arrests even if there might be rather good evidence pointing at the suspect at the time. There are so many murders happening that the police is apathetic about catching them only taking action when the evidence is conclusive. Population is openly hostile to all outsiders coming to the area and modernizers end up killed by some local or another. Modernizers keep getting killed, but new ones keep lining up to take the chance. All the villages are pictoresque, because no one can modernize anything without being a target for murder. The murder rate is sky high, but there is no mass exodus to safer areas. So, could these events be connected? All the murders in Midsommer keep property values down, making the area attractive to new entrepeneurs. The people of Midsommer are conservative and are quite happy to keep it that way. If someone snaps, kills a few outsiders and prevents change, it's just good for the locals and their way of life. The constant murders keep the population down and prevents immigration from exploding the property values like it does in the rest of Britain. With low population pressure there is little pressure to modernize and attract immigrants. All the killers are singular nutters with their own obscure motivations to kill, but the ultra-conservative, old-time countryside idolizing culture in Midsommer seems to be rather permissive of dealing with the threat of change by the act of murder. So maybe the constant epidemic is the Midsommer's defence mechanism against the outside world coming in and assimilating them into modern Britain. Under those circumstances the local people have a very low threshold to start killing. Midsomer is The Island; a large scale prison designed to detain Tom Barnaby.
In one episode Barnaby is revealed to be a former member of the intelligence services. He learnt too much and was eventually shipped out to 'Midsomer' to live out his days harmlessly. It explains the county's ludicrously high murder rate, and how many people reappear regularly in different roles. Barnaby is aware but happy to play along in an idyllic existence. It also explains why the high numbers of gruesome murders he sees doesn't affect him; he knows they're all fake. Life on Mars (2006) and Ashes to Ashes (2008) were set in Purgatory. Maybe Midsomer Murders is set in Hell.
All the cases of You Look Familiar in the show are the result of Uncanny Family Resemblance
In at least one case it was made explicit that two characters played by actors who had been in the show before were playing the sister and cousin, respectively, of their previous roles. Not too far fetched that a place with as many murders and inbreeding as Midsomer County has a pretty limited gene pool.