Zika virus particles (red) seen using transmission electron microscopy (TEM), a technique whereby a beam of electrons is transmitted through an ultra-thin specimen. So far, 58 countries and territories around the world have reported continuing mosquito-borne transmission of the virus, according to the World Health Organisation (WHO). The virus is transmitted when an Aedes mosquito - which also transmits diseases such as dengue - bites someone who is already infected. When that mosquito bites a second person, he could then be infected with Zika. The virus can also be spread through sex. Symptoms for Zika are similar to those for dengue. They include fever, rashes, joint or muscle pain and headaches. Preliminary research has led the WHO to conclude that Zika can cause microcephaly - or abnormally small heads - in unborn children if the mothers are infected. Zika also causes Guillain-Barre syndrome, a rare condition in which a person's immune system attacks the nerves.