Colonists investigating whether a new planet will support them, or whether it may contain some deadly disease, are met by a member of a lost colony on the planet, Pat Mead.
Pat returns to the ship with them, and undergoes the necessary decontamination procedure, including inoculation with a 'cureall' that destroys any non-human cells. However, shortly after he arrives, a sickness begins to spread. The only disease that Pat is aware of is the melting sickness, a plague that ravaged the colony centuries ago, but was apparently eliminated.
It transpires that the disease is not caused by any microbe--is not even properly a disease. The treatment which, centuries ago, adapted Pat's ancestors to the planet by forcibly evolving their cells to consume native foods had worked too well: the melting sickness is caused by Pat's cells--human cells--invading and destroying the cells of the unfortunates who contract the disease.
The colonists are able to beat the disease by entering regeneration tanks set to treat cancer: destroying and regrowing the whole body (except the nervous system), which results in the whole body being replaced by Pat Mead's cells. The men all come out looking like clones of Pat Mead--the same as all the surviving male colonists. The women were unaffected, Pat's cells only infecting the men, but in the end, the ship is visited by a group of the colonists including Patricia Mead...
A similar device is used in Unnatural Selection.