[ClearCase_guy]

The first outbreak in Marburg was traced to green monkeys imported from Africa.

I see two possibilities: these kids living in Africa came into contact with monkeys living in Africa.
OR
A nefarious scientist infected a batch of vaccines, in an effort to unleash a biological holocaust on the people next door, so that their mineral resources could be seized.

I'm going with Option 1.

[ordinaryguy]

Or one child had the virus and needles were re-used, infecting the rest of the children.

Or the vaccine was accidently contaminated.