The boy was admitted to the hospital yesterday after he had an epileptic fit, said inspector V Yadagiri Reddy of Nallakunta police station.
He was put on saline. According to his family members, the bottle of saline solution had traces of fungus.
The boy's uncle today lodged a complaint with police.
Doctors failed to check the bottle before putting the boy on saline drip, and though the bottle was removed after the fungus was noted, some fluid did pass into his body, the complaint said.
A case under IPC section 336 (act endangering life or personal safety of others) was registered against the RMO and other doctors, inspector Reddy said.
"We have seized the saline bottle and sent it for examination," he said.