Brian Jetter was on life support, a healthy 40-year-old suddenly battling pneumonia and sepsis, and a slew of tests had failed to find the cause.

Mystery illnesses like this kill thousands of people each year when germs can't be identified fast enough to reveal the right treatment. Now genetic tests are helping to solve these cases.

One finally was used to search Jetter's blood for bits of non-human genetic material from viruses, fungi and the like. It detected unusual bacteria that probably got into the Connecticut man's lungs when he choked and accidentally inhaled bits of a burger weeks ear...