Story highlights At least 225 deaths reported in Angola

Yellow fever starts with a mosquito bite on an infected monkey

If the mosquito bites a human being, the virus is transmitted

(CNN) A yellow fever outbreak that killed hundreds in Angola appears to be moving to neighboring Congo, where it has left at least 21 people dead.

The Democratic Republic of Congo reported the deaths this week, the World Health Organization said in a statement.

It said between January and March, at least 151 people were suspected of having the disease in the Congo.

Some of the cases were detected in areas bordering Angola and "were imported" from there, the organization said.

At least 225 deaths have been reported in Angola as of this week , the nation's worst yellow fever outbreak in three decades. Most of the cases have been in the capital, Luanda.

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