Serious systemic fungal infections and parasitic infections usually only develop in individuals who are immunocompromised. These individuals have a weak immune system. The body cannot respond quickly to infections since their immunity is not very well developed.
I would say energy myself, and this is because energy plays a key role in all of these. With trophic levels energy is being transferred amongst the levels. Food chain's is a map of how the energy is passed along the chain. Food web is a web of how energy is transferred, and this is why I would say energy is a keyword.
Some characteristics that is shared is a soft body and an outer shell to protect their body.
