The immune system

Hanta virus, SARS, H5N1 (bird flu), and possibly the original 1918 Spanish Flu, all can induce the body to produce a "cytokine storm", and a healthy immune system can be a liability for you. http://en.wikipedia.org/wiki/Cytokine_storm

It is seen to a small degree in pulmonery anthrax too, but there is some speculation that it's a common feature of many rapidly fatal infections, and the tendency to storms might be genetic ... SARS fatality seemed to be genetic because groups of blood relatives died.

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Come to think of it, many sets of parent and children were wiped out by the Spanish Flu ... so that adds to the familial tendency

There's a well-established theory around now, that sufferers of ulcerative colitis and Crohn's disease (which are auto-immune) diseases do so because they have such high levels of IgE (a class of immunoglobulin that patrols mucous membranes) and the IgE attacks "self" cells because it just gets out of hand with nothing better to attack. That's why in Europe at least now, you can get a prescription for tapeworm eggs (for a species that only survives in humans for a very limited period) to take. The tapeworms give the immune system something REAL to fight against, as it was always meant to do. People taking the medication tend to go into remission of symptoms while taking it.

Lupus is another example of an auto-immune disease. Perfectly healthy people, whose immune systems turn on themselves.

Have fun.