"Uexplained" could include a lot of people who die at a ripe old age, but without formal autopsy to determine the exact cause of death.
Theoretically, true, but practically speaking, there are few death certificates filled out "unknown" as a cause of death. Certainly not a million out of 2.4 million deaths a year to match the 2 million unexplained deaths since 2008 mentioned in the article.
It could also include deaths in car crashes, where the cause is not proved, and could be either driver error, extreme weather, or mechanical failure.
Except this news story is about some "unexplained lung disease". With all the attention focused on H1N1 and every cough and sneeze lately, could we really have missed this many people dying from some lung disease? Well, maybe if they were all driving a certain Japanese brand of car when they crashed driving themselves to the hospital...ooooooh, that's a juicy new conspiracy theory!