The table below classifies four disease-causing organisms (pathogens) by the broad group they belong to.

Pathogen group	Example organism	Disease caused
Bacteria	Salmonella	Food poisoning
Virus	Influenza virus	Flu
Fungus	Trichophyton	Ringworm (a skin infection)
?	Plasmodium	Malaria, transmitted by the bite of an infected female mosquito

Which pathogen group is represented by the question mark in the table?