A lot of infectious diseases are seen as a third world problem.....ie poor governments who will not pay for expensive drugs. As a result of this, a lot of the bigger drug companies steer clear of working on these areas. They can make a lot more money selling cancer treatments to the first world instead.
Thankfully, some companies do appear forward thinking. For example, Novartis has set up an infectious disease research facility in Singapore (http://www.nitd.novartis.com/) that plans to offer drugs against a few infectious diseases at cost. Whether they actually succeed is another matter, but they appear to be sinking a fair amount of time and resources into the venture. Maybe some of the other large companies will follow suit. Dengue spreading to the first world should help focus their minds a little. If they think they can make a healthy profit by selling treatments in the west, then they will work on it. Their shareholders will insist.