Thirteen high school students in Hanoi, Vietnam, are suspected to have contracted the swine virus, the health ministry said. Samples of the 13 patients have been taken for testing, and two cases of the swine flu have been confirmed, doctors told Tuoi Tre News. The diagnoses came after 20 students were hospitalized with fevers, coughs and respiratory problems. "Weather in winter-spring is favorable for rapid avian flu outbreaks that can lead to an epidemic, especially in closed and crowded places such as offices, schools, and public locations," said Tran Dac Phu, who heads the Health Ministry's Preventive Health Department. The health ministry is warning Vietnam residents to take measures to prevent the strain of H7N9 avian flu from spreading into the country from China. "The epidemic can penetrate into Vietnam at any time since there is a large volume of people and goods, including poultry, that travels between the two countries at the border everyday," said Phu, head of the department. The H7N9 bird flu virus has infected 40 people so far in January, Tuoi Tre News reported.