Although Zika and dengue are considered different virus “species,” they are so closely related that the immune system treats Zika just like another version of dengue, report researchers. Their latest study shows that pre-existing immunity to dengue virus modulates the magnitude and breadth of the immune system’s T cell response to Zika. Dengue comes in four serotypes and infection with one serotype does not lead to lifelong immunity to the other three, and in reality, the main risk factor for severe disease is a secondary infection with a different serotype. The method is thought to be "antibody-dependent enhancement" or ADE for short: antibodies developed against one serotype that are unable to prevent infection with another serotype, but instead exacerbate the second infection by helping the virus slip into immune cells. With recent studies having shown that antibodies isolated from dengue-infected donor could have both potent neutralizing activities against another serotype and induce ADE, researchers explored whether similar cross-reactivity exists on the T cell level. Since Zika and dengue co-circulate in many regions of the world, it is critical to start exploring the protective versus potentially pathogenic influence of T cells induced by prior dengue exposure on Zika infection. This was tested in a mouse model, and it was found that immunization with Zika-specific or Zika/dengue cross-reactive peptides protected mice against subsequent infection with Zika. Further experiments also revealed that the protection was dependent on functional T cells from previous viral infection. Another possibility in tackling the Zika outbreak, but one that still needs further refinement and effective implementation methods to prevent undesired effects from using a virus to treat a virus.