Need help on making a plant.

So i made this plant "coronavirus" with a simple image i found on google in Mcreator, when a entity touches it, it will give potion effects: poison,weakness,nausea. So when the entity stops touching it they will leave a trail of it when they walk. So i need to create some sort a variable to tell such as: If contaminated is = to TURE then place the virus block each time the entity walks. So how can i make that variable? I know that variables are only local.