Beyond obvious mood spoilers such as poor hygiene and bad breath, we each have a distinct natural scent that appears to guide us toward choosing a partner with compatible DNA. Scientists have found that women prefer the scents of men with a complementary set of genes that code for the immune system.

CNN: The science of kissing

The global burden of illness owing to dirty water, bad or non-existent sanitation and poor standards of hygiene is remarkable, according to the studies cited by Mr Rijsberman.

ECONOMIST: Economics focus