The death toll in a Legionnaire's disease outbreak in Portugal has climbed to eight, officials have announced.

The death toll has climbed to eight in a Legionnaire's disease outbreak in Portugal which the World Health Organisation has called a public emergency, officials say.

The latest victim is the second woman to have died since the first case appeared on November 7. Few details were released about her on Sunday, including when she died.

The Portugese government has said the outbreak was very likely caused by bacteria found in the refrigeration system of a fertiliser factory in Vila Franca de Xira, about 30 kilometres north of Lisbon.

At least 317 people have been infected since the first case was discovered, but on Friday the head of Portugal's health board Francisco George said the risk of new infections had "returned to normal".

The disease is not contagious and cannot be spread directly from person to person, but can multiply in water and air conditioning systems, including humidifiers, whirlpools and spas.

It is usually caught by breathing in small droplets of contaminated water.

The WHO called the outbreak a "major public health emergency" on Tuesday and the Portugese government has called it the third biggest in the history of the disease.

Environment Minister Jorge Moreira da Silva said the Adubos fertiliser factory was tested to see if there was negligence in maintenance and if any "environmental crime" had been committed.

Legionnaire's disease was discovered in the United States in 1976 after a convention of the American Legion, a military veterans group, at which 29 people died.

Copyright AAP 2014.