Lisa Hernandez, age 11, of Staten Island, New york, for her question:

How does a cholera epidemic spread?

The World Health Organization is faced with cholera outbreaks in at least 18 countries. In the past, this serious epidemic could have become a global pandemic, fatal to most of its victims. Modern medicine can reduce the fatalities to two per cent and vaccines can provide travelers with limited protection. Many countries have closed their borders to persons, food and other items from infected areas. Quarantines and the treatment of victims are both essential to halt the spread. But the real threat is much harder to control.

The dread disease is caused by Vibrio cholera, a bacillus that survives only a few hours without moisture. It can live only inside the human body or in water contaminated by human wastes. For example, it thrives unchecked around the Ganges River, where 3,000 cholera victims die every year. An infected traveler can spread the contamination far and wide. In the past, he carried the infection in his body for three weeks. However, E1 Tor, a new strain of the bacillus makes a person contagious for three months. The present threat is serious. But, at last we know that the fatal blow to the dread plague is decent sanitation with uncontaminated water supplies.