A new method that can precisely track the replication of yellow fever virus in individual host immune cells has been developed by researchers. The technique could aid the development of new vaccines against a range of viruses, including Dengue and Zika. Firstly, a bit of background on yellow fever: Yellow fever virus is a member of the flavivirus family that also includes Dengue and Zika virus. The virus, which is thought to infect a variety of cell types in the body, causes up to 200,000 cases of yellow fever every year, despite the widespread use of a highly effective vaccine. The vaccine consists of a live, attenuated form of the virus called YFV-17D, whose RNA genome is nearly identical to the virulent strain. This slight difference in the attenuated virus' genome may subtly alter interactions with the host immune system so that it induces a protective immune response without causing disease. In investigating the spread of yellow fever, researchers at Princeton University adapted a technique, called RNA Prime flow, that can detect RNA molecules within individual cells. They used the technique to track the presence of replicating viral particles in various immune cells circulating in the blood of infected mice. Mice are usually resistant to the virus, but researchers found that even the attenuated YFV-17D strain was lethal if the transcription factor STAT1, part of the antiviral interferon signaling pathway, was removed from mouse immune cells. The finding suggests that interferon signaling within immune cells protects mice from yellow fever, and that species-specific differences in this pathway allow the virus to replicate in humans and certain other primates but not mice. Using their viral RNA flow technique, the researchers determined that the virus can replicate inside certain human immune cell types, including B lymphocytes and natural killer cells, in which the virus has not been detected previously. By applying this technology to other viruses, we can potentially better understand the dynamics in the epidemiology of the increasingly prevent ARBO viruses.