Any number of opportunistic infections. Much will depend on your personal immune status and what organisms exist in your tank, but there are many spp. of
Vibrio and related organisms, not to mention the number of
Dinoflagellata spp. that can utilize human tissue and body temp as a growth medium. Keep in mind that our bodies are not that far from seawater... ...we are lucky to have those phagocytic WBC's and misc. immune antibody marker systems as part of the home team.
If you have an infection that develops and does not respond to normal emperic therapies, or a wound that will not close and heal, or you develop a fever that will not abate, mention to your practitioner that you have a
marine aquarium, as there are many organisms that will not be considered as potential infective agents if you have an oozing wound in a locatioin more than 50 miles from the Ocean.
Interestingly enough, one of the theories of the evolution of plants involves the "infection" of eukyrotic organisms by certain primitive spp. of cyanobacteria. These
Cyanobacteria spp. further evolved within the eukaryote into a symbiotic organelle that we now recognize as chloroplasts.
My 2 cents US.