It's the microscopic scourge of Durand Eastman: E. coli.

Last year, swimming was allowed at Durand 48 days and prohibited for 30 days. The leading cause for closure was heavy rainfall, but bacteria were second, accounting for 38 percent of the shutdowns.

The County Health Department is trying to locate the source of the E. coli, said county Public Health Director Dr. Andrew Doniger during a recent Legislature committee meeting. But it's not been an easy task.

"I'm on the side that there isn't going to be a single source," Doniger said.

The bacteria could be carried in runoff from "highly developed residential areas with steep slopes," or from local sources like animal waste or failing septic systems, he said.

Health Department staff is testing streams that run near the beach to try to pinpoint the source of the contamination, Doniger said. They are also doing DNA fingerprinting of the E. coli to determine whether it's of human or animal origin, he said.

In the latter case, failing septic systems could be a contributing factor - many homes near the lakeshore do not have access to public sewers.

The Health Department is preparing brochures to educate homeowners about failing septic systems, Doniger said.

The one place that the bacteria are definitely not coming from, Doniger told legislators, is the Van Lare treatment plant. The discharge point is farther out in the lake and bacteria levels measure near zero out there, he said.

"That has been analyzed very carefully," Doniger said.

The E. coli that Health Department officials are testing for is different than the E. coli associated, for example, with undercooked hamburgers.

"This is more of a broad class of organism that is easily tested for and received by the EPA as an indicator of bacterial problems," says John Ricci, spokesman for the Health Department.