Dear Art et al.,

You asked, "Is there any mechanism that prevents marine creatures from
transmitting diseases?"

Well, yes and no. This is another of these very large topics, and I just
know a bit here and there from courses in geochemistry and years of reading
Discover magazine, but you are welcome to them:

Some of the ions that occur naturally in seawater (e.g., iodate ion) are
toxic to many bacteria. So pure seawater is not only relatively free of
bacteria, but may even kill some of the bacteria on the surface of your
skin. And sewage discharged into seawater is not as dangerous as sewage put
into rivers, because kinds of the bacteria are quickly killed. Also,
bacteria are generally concentrated along surfaces, such as the outer
surfaces of sand grains, clay particles, and living organisms. They may be
present in great abundance there.

Most bacteria, from any environment including the marine realm, are not
dangerous to human beings. They wouldn't know what to do with your flesh if
you offered it to them, Art, since they are very simple and have very
particular preferences. Most soil bacteria will not even grow properly in
agar-agar, the biologists' preferred medium. Also, your body can
successfully fight off most microbes without trying hard. Otherwise people
would be dying of all kinds of weird diseases all the time, since microbes
are everywhere.

The diseases that tend to be transmitted most readily are those that affect
related species. Diseases of sheep may be quite dangerous to goats, not as
dangerous to horses, even less dangerous to birds, and not at all dangerous
to snails. Shared diseases and parasites form some of the best evidence for
shared ancestry. Complications arise in cases where diseases mutate and hop
from species to species (for instance, there is good evidence that humans
acquire new strains of influenza from domestic ducks and chickens). Life
also gets interesting in cases where the normal life cycle of parasites
includes stages that live in different animals, such as freshwater snails
and mammals. Nature is full of special cases, but let's stick to the big
picture for now.

It's currently thought that humans have been acquiring some new and very
dangerous diseases from other mammals. We didn't have a lot of contact with
them before, but now there are more people living in the rain forests, and
they hunt for "bush meat" and also for specimens for labs and zoos. HIV
seems to be derived from primates. Mad cow disease apparently originated in
sheep. We don't know what animal harbors Ebola virus in nature. People in
medical circles are getting increasingly uneasy about using medical products
derived from people and other animals for this reason. That millionth
transfusion or transplant could have some nasty surprises in it!

Anyway. Compare this with handling marine snails and clams. Although
handling living mollusks certainly means contacting their parasites and
bacteria and viruses too, the risk of disease is very low. Most microbes and
parasites living in mollusks would not be able to invade a human body and
thrive there. Some might cause problems from sheer toxicity, e.g., red tide.
The risk is somewhat greater from handling marine fish (especially from
eating raw fish), but fish are not very closely related to us and even that
risk is low. The risk from domestic animals -- and in some cases from land
and freshwater snails -- is much higher for us than from marine creatures.

Andrew K. Rindsberg
Geological Survey of Alabama