It turns out there were about 70 cases in the Institute's archives that represented autopsies of 1918 flu victims that contained tissues that were formalin fixed and paraffin imbedded as well as accompanying glass slides and clinical records. So we started the project by pulling half of them randomly.
The first thing I did was to examine the cases under the microscope and try to learn something about the pathology of the 1918 victims. And looking at the clinical records and the slides from these cases as well as what's known about the pandemic in general, people died in different ways. Some people died very quickly after coming down with flu symptoms in 1918, which was something that was really very unusual about the 1918 epidemic. Some people would die within two or three days after the onset of symptoms, and very unusual pathology in which their lungs were just filled with blood, pulmonary hemorrhage where they would just fill with fluid, that is pulmonary edema, and they would basically drown, and with very little inflammation. And this kind of pathology was really unique.
The other really odd thing was that people who died with this kind of pathology tended to be young, healthy people. Influenza viruses normally only kill the elderly or very young, people who basically don't have normal immune systems. So that was a very unique feature of the 1918 flu.
The reason that this was important is that influenza viruses are known to replicate extremely quickly. That is, they make copies of themselves in the cells that line the lung and then after about five days, they leave that host after having replicated and then infect other people. So after about a week's time, even under modern circumstances, it becomes impossible to recover the virus. So we wanted to select four cases which the clinical course from onset of symptoms to death was under a week.
One of those cases still contained genetic fragments of the 1918 virus, and that was a case that had really good histologic evidence, that is under the microscope, of primary influenza pneumonia.