Forty-two million people worldwide are infected with HIV, and the vast majority of them live in the developing world, with little access to sophisticated labs that can monitor their immune-cell levels-measurements critical to determining their need for and response to drugs. Researchers from the University of Texas at Austin and Boston’s Massachusetts General Hospital are developing a portable immune-cell reader to fill this gap. At the heart of the device is a microchip that filters white blood cells out of a few drops of blood and stains the key ones red, green, and yellow. A digital camera then takes a picture of the cells, which software analyzes to determine the counts of each cell type, indicating how well the immune system is holding up. Though the current prototype is the size of a desktop computer, the researchers aim to produce a handheld version within the year. Ultimately, they hope each test will cost less than $3, compared to the $35 to $60 charged by conventional labs. Early trials of the system conducted in Boston and Botswana have been encouraging. The researchers say testers in Botswana liked the prototype so much they didn’t want to send it back.