It shares a basic structure with all flaviviruses: Genetic info in the form of RNA is surrounded by a fatty membrane, then encased in a protein shell with a 20-sided face. The protein shells are made of 180 copies of two different proteins, each composed of chains of different amino acids. Once inside a target cell, the virus breaks apart and forces the host to do its bidding, replacing the instructions coded into the cell’s DNA with those programmed by viral RNA.

Zika differs most from other flaviviruses at a spot thought to be crucial to the cellular break-in. At this site, a carbohydrate molecule — made of different sugars — sits on the virus’s protein shell. This spot where Zika differs, called a glycosylation site, actually protrudes from the shell of the virus. In other viruses, similar protrusions act like strangers offering candy, tricking the human cell into binding with the invader. Like other flaviviruses, Zika seems to have a unique smattering of amino acids around that area.