The main problem may not be in the floodwater. Diseases people normally harbour will spread more easily and cause more damage among the stressed people who could be living in shelters for months.

The most important vaccine for people in the disaster zone is the combination diphtheria-tetanus-pertussis vaccine.

The CDC is recommending the vaccination for everyone in crowded shelters, everyone under 18 displaced by Katrina wherever they are living, and for any displaced adult or relief worker who has not had a tetanus shot for 10 years.

But the federal stockpile contains none of this vaccine, a spokeswoman for the US National Vaccine Program Office told New Scientist. It is supposed to contain 10 million doses. This means relief agencies will have to acquire the vaccine on the open market, which could take time, and supplies might be limited.

Read (New Scientist)