Mumbai-based Sensex dipped nearly 1,300 points intraday after Karnataka reported India's first two cases of HMPV in babies. The index fell to a day's low of 77,959.95 from yesterday's close of 79,223.11 before marginally paring losses to trade at 78,200. Tata Steel, Kotak Mahindra Bank and Asian Paints were top losers on Sensex.