'Look at Iceland they've had very low immigration over the last 1000 years and have a very 'clean' Viking gene pool but have a 'very' healthy population.'

Actually you couldn't have chosen a worse example. Iceland has a disproportionate rate of genetically determined mental illnesses such as schizophrenia and alcoholism precisely because of the limited gene pool of the original settlers. Conditions that would normally have been 'bred out' by a constant flow of new immigrants and the interchange of people have persisted in the gene pool simply because of Iceland's remoteness.

The relative health of the Icelandic population is a recent occurrence as their country moved from a pastoral/fishing society. Reports as recently as the 19th Century talk of how the Icelanders had a marginal existence. Cheap energy, access to global markets and the post-war Scandinavian social model of government are responsible for the modern healthy Icelander.