WARSAW, April 28 (Xinhua) -- Polish Health officials reported on Tuesday afternoon that there are three suspected cases of human infection of swine flu in Poland, and the patients are under observation in hospitals in Warsaw and Elblag in northern Poland.

    "The three people with flu-like symptoms recently returned from Mexico," spokesman for the Chief Sanitary Inspectorate Jan Bondar told Polish news agency PAP. "They have been hospitalized but their condition is not serious."

    Swine flu is a respiratory disease that infects pigs and does not normally infect humans. But sporadic cases had happened, usually among people who had close contact with pigs. More than 150 people are thought to have died from the disease.

    On Monday, the World Health Organization (WHO) raised its global alert level from Phase 3 to Phase 4, signaling the swine flu virus was spreading from human to human in community outbreaks, but it did not declare a full-blown pandemic.