Ozma rescues Ranka from the research fleet that the Vajara destroy.
I thought the main difference between the two is Ranka's mother was given the virus by Grace, Ranka was born with the fold bacteria
Already inside her, which put them in the correct spot (gut). Sheryl was infected as a child by Grace, and that's why it's killing her, because the bacteria is not in the right location.
Also Ranka's memory only shows a small ship being destroyed, not the Research groups SDF class ship which is shown in the series.