Health officials banned swimming at private, man-made Lake Mission Viejo after at least 13 swimmers, most of them children, became violently ill.
Tests of the water, however, revealed nothing that could have caused the flu-like symptoms, and the lake was reopened.
Symptoms included vomiting, diarrhea and severe cramps, said officials, who remained at a loss to explain their cause.
The 125-acre lake is open only to residents of Mission Viejo who have joined the lake association. The lake was closed in April, 1984, after about 20 swimmers complained of rashes like flea bites caused by tiny water-born parasites. Birds carrying the parasites were shot and the lake water treated before the lake was reopened the following July.
Later boaters discovered that the chemical used to control water parasites was corroding the hulls of aluminum boats. The lake association agreed to pay for repairs.