[originaltext]　　(3) The World Health Organization has announced that the Ebola

　　(3) The World Health Organization has announced that the Ebola outbreak in Guinea is over. The epidemic began in Guinea in March 2014. The BBC’s global health correspondent (记者) Tulip Mazumdar reports. "The worst Ebola outbreak in history began in Guinea and the country is now the last of the most severely-affected nations to declare an end to the epidemic. This is really exciting news which cheers up the health workers all over the world. But there are warnings Guinea should prepare for further sporadic (不时发生的) cases. This is what people should focus on seriously. In Guinea, there will now be three months of heightened surveillance (监视). (4)Keeping Guinea free of Ebola will mean communities stay cautious, which health workers say is the biggest challenge."
　　3. What does the World Health Organization announce recently?
　　4. What is the biggest challenge of keeping Guinea free of the disease?