21.   Critics said the slaughter does not answer the key question of where the virus is coming from.

22.   Hong Kong officials suspected the virus came from the mainland, although this was not supported by World Health Organisation tests.

23.   Lina said he still doubts that the virus came from his farm.

24.   Researchers do not know where the virus comes from, but when it flares up, it kills humans so fast that outbreaks usually only last a few weeks.

25.   Researchers do not know where the virus comes from.

26.   Scientists do not know where the virus comes from, and there is no known cure.

27.   The disappointing reports suggest that the tough virus is coming back after being knocked briefly into submission, just as many experts feared it would.

28.   The virus soon comes back when people stop their drugs, but all measurable levels disappear when treatment resumes, and no resistant viruses have been seen so far.

29.   There is no known cure for Ebola, and doctors do not know where the virus comes from.

30.   They said it was premature to close down chicken farms in the region as the virus comes and goes with infected birds.