In this article the views of the ancient Greeks and Romans on the etiology of infectious diseases are assessed. It appeared that these views were remarkably correct in many respects: Hippocrates for instance believed that an imbalance in the humours preceded disease, while we know today that a malnourished body predisposes a patient to epidemic disease. Further acute observations were recorded during the plague which afflicted Athenians in the 5th century BC, when it was noted that the disease (probably smallpox) was spread by close contact with patients and that the same person never contracted the disease twice – the first description in Western history of acquired immunity. The ancients’ theories of miasmata and ‘seeds of disease’ in the air were the forerunners of what is today identified as pathological micro-organisms causing disease. Little progress in the study of the etiology of infectious diseases was made since Graeco-Roman times, in fact, in the 19th century it was still believed in London that infection was the result of ‘bad air’. The problem was eventually solved when in the 19th century Robert Koch, with the help of the microscope, discovered the pathogenic organisms causing infectious diseases. In many respects the scientific discoveries during the last two centuries merely confirmed the observations of the ancient Greeks and Romans made more than 2000 years ago.