A functioning immune system disposes of viruses through their lower alimentary.
If you checked your “deposit” today, you would find all sorts of lipid-envelope viruses; measles, mumps, chicken pox, polio, possibly smallpox.
It is the nature of the virus to replicate it self constantly.
Once immunity is achieved, these are captured by white blood cells and passed through the body without harms.
There is no reason not to believe that seagull immune systems work any differently.
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CDY, I trust you more than I trust the CDC...but that is damning with faint praise.
Assuming you read this in a credible scientific book or journal, is there any chance you picked one up from the 1930s?