I'm not too sure what CBT for a clam would involve. But clearly those clams were not maintaining a positive frame of mind and so were susceptible to becoming ill.
Tasmanian devils (cute but feisty marsupials) are being badly affected by a contagious facial cancer. (edit, I see now that the bivalve article mentions Tasmanian devils)
In the mid-1990s, the first signs were observed of the fatal and infectious cancer, Devil Facial Tumour Disease. Sightings of the Tasmanian devil have since declined by more than 70 per cent.
http://www.tassiedevil.com.au/tasdevil.nsf/thedisease/bd2717c762779ee8ca2576f1001d0110
Cervical cancer in humans has been shown to be caused by a virus (HPV). It seems to me that it is likely that these other 'contagious cancers' are caused by, or at least require, viruses or other pathogens that have not yet been identified. So, it's not that the cancer is contagious. Rather it is that the pathogen that triggers the cancer (or stops the body from effectively dealing with cancerous cells) is.
But I could well be wrong. Perhaps what those Tasmanian devils really need is to calm down with a bit of mindfulness training and to eat more vegetables.
Edit: I probably am wrong. Seems researchers think the Tasmanian devil cancer is caused by the transmission of the cancer cells themselves (allograft). And hamsters have a contagious cancer too.
From Wikipedia Devil facial tumour disease
Transmissible cancer, caused by a clone of malignant cells rather than a
virus, is extremely rare, with only two other known transmissible cancers—
canine transmissible venereal tumour (CTVT), which is sexually transmitted among dogs, and contagious
reticulum cell
sarcoma of the
Syrian hamster,
[77] which can be transmitted via mosquito bites of
Aedes aegypti.
[78] CTVT mutes the expression of the immune response, whereas the Syrian hamster disease spreads due to lack of
genetic diversity.
[79]
Certainly there is still much to be known.