Monk seals weren't around in any significant numbers until 15 or so years ago.  The seals have largely been absent from the main Hawaiian Islands since shortly after the first Polynesians arrived, so their return to the MHI means their first real exposure to cats and toxoplasma.  We have looked at a variety of blood values (including antibody levels etc) and contaminant loads etc. There is nothing to suggest that these seals are immune compromised. They are being exposed to a disease that they are not well-equipped to deal with.  It is a similar mechanism to when indigenous cultures are exposed to new diseases from foreigners...the results can be devastating.