This paper complements Amartya Sen's hypothesis on the world's "missing women" -- the idea that millions of women are missing from the world due to sexual violence and discrimination -- by offering a biological explanation for the sex disparity. This paper hypothesises that the sex ratio imbalance may also be due to the effect of Hepatitis B on birth ratios. In many Asian countries the ratio of male to female population is higher than in the West -- as high as 1.07 in China and India, and even higher in Pakistan. A number of authors (most notably Sen, 1992) have suggested that this imbalance reflects excess female mortality and, as a result, have argued that as many as 100 million women are missing." This paper proposes an explanation for much of the observed over-representation of males: the hepatitis B virus.