I don't normally go round quoting Norman Tebbit, but my curiosity was piqued and my sense of humour tickled by his speculation that Mugabe was displaying symptoms of the advanced stages of syphillis ...

... Private eye later referred to the article, pointing out the irony in Tebbit accusing anyone else of frothing at the mouth, but nonetheless it seemed as good an explanation as any of the other informed comments here.