"When scientists began creating adenoviral vectors in the 1980s, most worked with a particular kind of adenovirus called Ad5, which ubiquitously infects humans and causes the common cold. Researchers stripped Ad5 of the genes it needed to replicate and inserted those genes into genetically engineered cell lines. That ensured that the modified viruses could be grown only in these special cells in the lab. It also opened up space in the Ad5 genome for scientists to stitch in new genes of their choosing. Many scientists hoped to use Ad5 to deliver a human gene that could correct rare genetic mutations—an approach called gene therapy. Those efforts came to a grinding halt in 1999 when a teenage boy with a rare genetic liver disease died after receiving an injection of an Ad5-based gene therapy, which had been designed in James Wilson’s lab at the University of Pennsylvania. The large dose of 38 trillion viruses the patient was given sparked massive body-wide inflammation and sent his immune system into overdrive. After that, scientists mostly stopped using adenoviral vectors for gene therapy, in which the dose needs to be high to reach many cells of the body."