(CNN) -- The New York City Department of Health said Sunday it is investigating five patients on Staten Island who have or had symptoms of what appears to be viral encephalitis, but city health officials downplayed the possibility that they represented the harbinger of the outbreak of an infectious illness.

Health authorities were alerted Friday when the cases came to light.

"On Friday night, we thought we had something interesting, but we don't think we have anything interesting any more," Sandra Mullin, a spokeswoman for the city's Department of Health and Mental Hygiene, told CNN. "Our concern has lessened dramatically."

Of the five cases, one has since been released from the hospital and two were discovered to have underlying health conditions that explain their illnesses, she said.

"We're still looking at two people, but encephalitis does happen -- about 200 cases per year" in the region, she said.

Both patients are "quite sick," in critical condition, she said.

The initial cluster led public health officials to be concerned about "something in the environment" possibly causing the cases, she said.

"We were able to rule out West Nile and Eastern Equine Encephalitis," she said. "We're certainly not as concerned as we were on Friday."

Though the agency consulted with the Centers for Disease Control and Prevention in Atlanta, "we routinely talk to the CDC for far less."

"In our investigation this weekend, we have not found evidence of an outbreak, nor have we found other similar cases of unexplained illness beyond these two individuals on Staten Island," said Health Commissioner Dr. Thomas R. Frieden. "Nonetheless, the health department will continue to examine these illnesses to see if an explanation can be found."

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