Story highlights 28 people have become sick since attending a funeral in Sinoe County, Liberia

Symptoms include headache, diarrhea, vomiting and confusion

(CNN) An unexplained illness has claimed the lives of 12 people in Liberia since April 23, the World Health Organization reported Thursday. Twenty-eight people have fallen ill, including an unknown number of children, and three remain hospitalized at Francis Grant Hospital in Sinoe County.

The mystery began when an 11-year-old was admitted to a hospital April 23. The child had diarrhea and vomiting and was confused after attending the funeral of a religious leader the previous day, according to the WHO.

The child died within an hour of admission.

Symptoms of the illness include headache, diarrhea, vomiting and confusion, according to Fadela Chaib, a spokeswoman for the WHO. Health officials immediately tested the victims for Ebola, yellow fever and Lassa fever, but they were all ruled out.

All of the ill but one had attended a funeral April 22 in Greenville, Sinoe County. The exception was the partner of a man who had died in a hospital in the capital city, Monrovia. This partner became ill and died April 29.

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