How to cure contagious diseases via genetic sequencing without having any idea what causes the disease.
We know that for just about every contageous disease, there seem to be some people who are naturally immune to it. In addition, there are rare cases of families getting infected where some family members appear to have natural immunity to the disease while other family members do not.
If the environment is similar, some genetic factor conferring immunity is quite probable. By sequencing close relatives who do/do not have immunity, you could isolate pieces of DNA that were not the same. After sampling several rare cases, and cross-referencing the immune members unique DNA pieces, you could quickly zero in on a common area that all the immune people had in common with other immune people, but that was excluded from all the close relatives who fell ill. By starting with close relatives rather than the population as a whole, the work in getting to the sequence responsible for conferring immunity would go more quickly. This would be especially nice for new contagious diseases where the cause was not obvious.
Footnote: Authors note looking back from 2006: this is a bit out of date now as there seem to be researchers doing this all the time.