>  I take it that parasites of marine organisms are not readily transferred to
>  humans. Can you tell that a marine bivalve or gastropod is infected with
>  parasites just by looking at it, or is this a microscopic thingy?

Some parasites can make the host snail look and act more conspicuous, promoting the change that the snail will be eaten by the ultimate host.  However, I think that's just a soft tissue phenomenon.

Some parasites castrate their host.  Energy that would have gone into reproduction instead goes into growth, resulting in a giant individual.  This would be visible, but hard to prove the cause, in a fossil.  Likewise, a stunted individual might suggest an adverse reaction to parasitism, but it would be hard to prove.

>I would like  to know whether parasitic infections affect the shell in ways that could be
>  recognized in fossils, the way that drillholes and some other signs of
>  predation (and commensalism) are.

Parasitic infection suggests you're thinking of endoparasites, as opposed to something like the hipponicids that may bore through a host shell as parasites or kleptoparasites.

Pearls are a potential parasite response.  Paleonet some time ago had mention of occasional fossil cephalopods with little bumps all over the inside of the shell.  That could reflect a batch of parasites that hatched out in the wrong spot and were encased by the former host.  Perhaps careful sectioning of the bumps could test that.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

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