First, a heart attack certainly could be related to a viral factor, I would think. Not common, but certainly feasible. Unusual diseases are often missed by doctors. A woman who went insane and died in my city was not diagnosed by any of the top neurologists here with having CJD. Only once she went to the Mayo Clinic was she diagonosed with CJD. At least her case does not appear to be vCJD, the slight variant linked to BSE(Mad Cow Disease). She died, since there is no cure for any strain of CJD, and yes they call variations of prionic diseases, strains.
Having one passenger die of viral menigitis, and another of a heart attack on the same flight is suspicious,especially given the fact that they were young people. People jump to the conclusion of bioterrorism, and I doubt that very much. However, people really badly underestimate the danger of natural outbreaks of very deadly diseases.
I guess the good thing is our immune system is pretty good at fighting most things, except when the germ doesn't look like at all what we've already seen. Typically, people who die from these things are thought to have some immune issue. Maybe the two people who died on that plane had some sort of lifestyle issues: drugs, alcohol, extreme lack of sleep,... For that matter, I guess transatlantic travel probably doesn't exactly help our immune system, I would think.