BANTUL, Indonesia (AP) – Many of Indonesia’s 650,000 homeless earthquake survivors are living with deteriorating sanitary conditions, forced to wash with dirty water that infects wounds and spreads skin disease, doctors said Sunday.

Another peril loomed from a nearby volcano, which spewed lava and hot gases dozens of times on Sunday.

There was also concern about bird flu in the quake zone, as the number of Indonesia’s human deaths from the virus mounted. Some of the homeless have taken shelter in chicken coops that aid workers fear could contain the disease.

Despite the hardships of day-to-day survival, farmers in the quake zone were returning to their fields to pick food, under pressure to earn money for rebuilding their shattered homes. Farming communities were among the hardest hit by the magnitude-6.3 quake that struck last weekend.

Most of hundreds of thousands left homeless are living in makeshift shelters – often just plastic tarps – with no toilets or running water.

Doctors said wells and streams in many villages have become polluted because of the poor sanitary conditions.

“There are still many who are sick, some with skin diseases because of poor sanitation,” said Hendra, a government doctor traveling around the quake zone in a medical van. “The water for washing is dirty and many patients are not taking proper care of their wounds.”

The doctor, who like many Indonesians uses only one name, said health personnel still had not reached many villages in hilly areas.

U.N. spokeswoman Amanda Pitt said sanitation and providing clean water to the homeless “remain a key concern.”

More than a thousand aftershocks have hit central Java since the earthquake struck May 27, killing at least 6,234 people and injuring 30,000. Officials estimate that 135,000 homes were destroyed.

Indonesia is prone to seismic upheaval because of its location on the so-called Pacific “Ring of Fire,” an arc of volcanoes and fault lines encircling the Pacific Basin.

Mount Merapi, one of the world’s most active volcanos, spewed lava and hot clouds of gas and ash dozens of times Sunday, said Sugiono, a government scientist. The volcano is north of Yogyakarta, the main city in the quake zone.

Merapi’s lava dome has swelled since the quake to 330 feet, raising fears that it could collapse, officials said. That could send searing-hot clouds of gas and debris pouring down the slopes into inhabited areas, the government volcanology center warned.

Surono, a senior government volcanologist, said a collapse wasn’t inevitable, but if it occurred it would be “very dangerous.”

“Let’s pray that this does not happen,” he said.

Bird flu was another possible threat to quake survivors, aid workers warned Sunday. At least 37 Indonesians have died from the virus, with World Health Organizers confirming Sunday that bird flu killed a 15-year-old boy who died last month.

“We are concerned that people using poultry sheds as shelter are at risk from avian flu and possibly salmonella infection,” said Dr. Yolanda Bayugo, health director in Indonesia for British-based aid group Merlin.

Meanwhile, farmers in the disaster zone donned straw hats and headed back to their rice fields Sunday. Others picked fruit, harvested maize or chopped down bamboo to sell, hoping for money to reconstruct their homes and lives.

“We have to go back to work. Otherwise we can’t eat or rebuild,” said Sri Supati, harvesting rice Sunday at a communal paddy field surrounded by crushed homes and piles of debris.

“We have waited for nearly a week, but no help has come,” said the farmer from Kerujukan in the hardest-hit district of Bantul. Nearby, several villagers were breaking down the remains of half-fallen walls, while others carted off bamboo to make temporary shelters.

The international relief effort has picked up pace in recent days, although aid has yet to reach some remote areas. The United Nations has appealed for $103 million for recovery efforts over the next six months.