The risk of foot-and-mouth disease spreading to humans is "extremely rare", according to the director of the National Disease Surveillance Centre.
Dr Darina O'Flanagan said there was "definitely a species barrier" preventing the spread of the virus between susceptible animals and humans.
Two more suspected cases of foot-and-mouth in humans were being investigated in the UK last night, a day after it emerged a slaughterman in north Cumbria was undergoing tests for the disease.
Dr O'Flanagan said the UK has only one recorded case in the last century of a human with foot-and-mouth disease, and there are only a handful of cases worldwide - even in areas, such as parts of Asia, where the virus is endemic.
Dr Patrick Wall, chief executive of the Food Safety Authority, said people would have to be exposed to a large amount of virus to contract the disease.
He said those who have contracted it have been working with the virus in laboratories, or have been working with infected animals, whose lesions would contain a large concentration of the virus.
Of the tests on the slaughterman in Cumbria, a spokesman for the UK Prime Minister, Mr Tony Blair, said: "My understanding is that he was moving the decomposing carcass of a cow, when that carcass exploded, and the fluid went into his mouth."
Humans cannot contract the infection by eating meat or drinking milk from infected animals. It is "not a public health threat", said Dr O'Flanagan.
Symptoms, which are similar to a mild 'flu, include a sore throat, high temperature, and blisters on the hands and feet. The term of the disease is very short-lived, said Dr O'Flanagan, fading within a few days.