The Zika virus is likely to spread across nearly all of the Americas, the World Health Organization has warned.The infection, which causes symptoms including mild fever, conjunctivitis and headache, has already been found in 21 countries in the Caribbean, North and South America, reports BBC.It has been linked to thousands of babies being born with underdeveloped brains and some countries have advised women not to get pregnant.No treatment or vaccine is available.The virus is native to Africa and was first found to be spreading in the Americas in Brazil in May 2015.The lack of any natural immunity in the Americas is thought to be helping the infection to spread rapidly.