In 1854, John Snow wrote on the outbreak of cholera in London:1

“the most terrible outbreak of cholera which ever occurred in this kingdom is probably that which took place in Broad Street, Golden Square and the adjoining streets, a few weeks ago. Within two hundred and fifty yards of the spot where Cambridge Street joins Broad Street, there were upwards of five hundred fatal attacks of cholera in ten days”.

An outbreak is usually thought of as a sudden localised increase in disease incidence. Classically, outbreaks are the outcome of infection but, increasingly, non-infectious agents are identified as their causes. These may be newly introduced without recognition of the associated risk or a well recognised risk to which the level of exposure is not appreciated. Several examples have been reported in recent years: an outbreak of obliterative bronchiolitis in the workforce of a microwave popcorn factory in USA probably caused by diacetyl, a volatile agent for flavouring butter;2 outbreaks of organising pneumonia in Spain and in North Africa in textile spray workers which, in both instances, followed the introduction of a textile dye whose chemical formulation had been changed;3 and outbreaks of lymphocytic bronchiolitis and peribronchiolitis in workers in …