Serum and blood samples of all six suspected patients from Kerala were found to be negative for the brain-damaging Nipah virus, top health ministry officials said on Thursday, as experts are yet to locate the primary source of the infection.A 23-year-old engineering graduate was tested positive for the virus in Kochi on Tuesday. Six people, who showed symptoms of Nipah, have been shifted to isolation wards and 311 others who were in contact with the patient kept under observation to stop the disease from spreading.The Aster Medcity Hospital in Kochi, where the Nipah patient is admitted, released a medical bulletin saying he is conscious and eating but has mild disorientation.State health minister KK Shailaja is expected to meet media in Kochi at 10am.Also read: No fresh Nipah case as Kerala fights hard to contain any outbreakThe only worry now is the source of the virus, which is yet to be located. Health officials scanned the places the affected engineering graduate had frequented in the last two weeks but have failed to find either fruit bars or infected pigs.Nipah virus is transmitted to humans through direct contact with infected bats, pigs or other people. Also read: All you need to know about the Nipah virusSymptoms of Nipah, which include fever, headache, myalgia or muscle pain, vomiting and sore throat. These symptoms can be followed by dizziness, drowsiness, delirium, neurological deficits and breathing difficulty as the disease progresses. There are no drugs or vaccines that can combat the virus, which has a morbidity rate of more than 70%. Fruit bats, better known as flying foxes, are natural hosts of the virus, which killed 17 people in Kerala last year.