The US military has discovered more suspected shipments of live anthrax than previously thought and has ordered a probe on laboratory practices that are supposed to inactivate the bacteria.
The Pentagon said 11 states in the US, Australia and South Korea received the "suspect samples". It added that there was no known risk to the general public and it posed "an extremely low risk to lab workers."
So far, four US civilians have started taking precautionary measures that include anthrax vaccine, antibiotics or both.
Twenty-two people at a US Air Force Base in South Korea have also began taking preventive measures although none of them has shown signs of exposure, officials said.
Here are five things you need to know about anthrax:
It is a serious bacterial disease
Anthrax is an infection caused by bacteria called Bacillus anthracis. The bacteria can form spores that can survive in harsh conditions. These spores can be found naturally in soil and they cause anthrax when they enter human bodies.
Anthrax can be fatal if left untreated. It is not contagious but its spores can be inhaled, ingested or transmitted through lesions on the skin.
A blood culture test is needed to confirm anthrax exposure.
Symptoms include skin ulcers, nausea, vomiting and fever, and can cause death if left untreated.
It is a biological warfare agent
Anthrax has been used as a weapon in the past. In 2001, powdered anthrax spores were deliberately put into letters that were mailed through the US postal system. Twenty-two people, including 12 mail handlers, got anthrax, and five of these 22 people died.
Stored as dry powder
It can be stored as a dry powder and loaded in a freeze-dried condition or disseminated as an aerosol.
Last year, an outbreak of Anthrax in a village in the Indian state of Jharkhand reportedly killed seven people. Officials traced it to a cow and found out that people had touched the dead cow or ate it.
Sources: BBC, The Guardian, Global Research, LA times, The Sydney Morning Herald