Another 41 people in the southwestern province of Sichuan were hospitalized with symptoms that include high fever, fatigue, nausea and vomiting, and "became comatose later with bruises under the skin," the official Xinhua News Agency said.
Twelve were in critical condition, Xinhua said.
The illness likely stems from streptococcus suis, a bacteria that is usually spread among pigs, provincial health official Zeng Huajin was quoted as saying by the China Daily newspaper.
"I can assure you that the disease is absolutely not SARS, anthrax or bird flu," Zeng said.
China is sensitive to such public health threats after criticism of its handling of severe acute respiratory syndrome, which first emerged in the country's south in 2002.
The government was widely criticized for its slow response to pleas for cooperation and information about the disease, which killed nearly 800 people worldwide before subsiding in July 2003.
China also is trying to contain an outbreak of avian flu in its west, where thousands of migratory birds have died in recent weeks.
A man who answered the phone at the Sichuan health bureau on Monday said 17 people have died from the mystery illness and another two have recovered. He refused to give his name.
The cause of the deaths was under investigation, he said.
A woman who answered the phone at the Ziyang No. 1 People's Hospital, where most of the patients were being treated, hung up when asked about the cases.
The symptoms reported "seem consistent" with streptococcus suis, said Bob Dietz, a spokesman for the World Health Organization's Western Pacific region in Manila.
"We don't think we've seen numbers on this scale before, but it might be because of a heightened surveillance system," Dietz said.
So far, China has kept the agency informed "in a timely manner," he said.
The last major pig-borne epidemic occurred in Malaysia, where 265 people were infected with the Nipah virus between 1998 and 1999.
Some 105 people died and nearly a million hogs were slaughtered before the outbreak was controlled. The virus is capable of infecting a variety of animals, and is lethal to about 50 percent of human patients, causing severe encephalitis.
The Chinese ministries of health and agriculture sent a special team last week to Sichuan to assist in the investigation, treatment and control of the outbreak, the China Daily said.
Xinhua said medical experts believe the mystery illness in Sichuan "is not spreading further among humans," and that there were "no obvious signs of (an) epidemic."
Local governments as well as health and agricultural officials have set up special teams to try to investigate the illness, it said.
Shanghai's Oriental Morning Post newspaper said the patients were between 30 to 70 years old; one was a woman.
The victims suffered from poisoning-related shock syndrome and were acutely infected, an unidentified employee at a hospital treating the patients said in a telephone interview aired Sunday on Hong Kong's Cable TV.
The son of one victim told Cable TV his father fell ill after slaughtering a sick pig and eating some of it. The names of the son and victim were not given.
Pigs in the area had been infected with streptococcus bacteria, which is common in domestic animals, the hospital worker said. It was unclear whether the sick sheep in the area were infected with the same bacteria.
Also Monday, two supermarket chains in Hong Kong stopped the sale of frozen pork from Sichuan as officials sought to assure the public that the mystery disease did not pose a threat to the territory.