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You could place the disease in the app as an item or spell, it does not matter that much, just make sure you mark it in such a way you can easily find it.
Have a look at this page (https://roll20.net/compendium/dnd5e/Diseases#content) for some example diseases, in the end it is up to the DM, but I usually make sure that the PC has a decent chance of getting rid of it (to find it in time) by making the effects of the disease more and more visible when time passes.