Modifiers matching intruder:

• harmful,
• unwanted,
• outside,
• pesky,
• nonhuman,

Simple categories matching unwanted:

• item,
• interference,
• input,
• pest,
• emotion,
• bacteria,
• particulate,
• material,
• software,
• organism,
• entity,
• state,
• solid,
• message,
• contamination,
• activity,
• intermediate,
• instrument,
• incident,
• souvenir,
• furniture,
• effect,
• email,
• fuel,
• insect,
• portion,
• substance,
• plant,
• panel,
• disturbance,
• species,
• filler,
• animal,
• content,
• particle,
• medium,
• artefact,
• solicitation,
• fabric,
• larva,
• condition,
• container,
• layer,
• odor,
• nuisance,
• stain,
• code,
• hardwood,
• reptile,
• tissue,
• variation,
• effluent,
• light,
• residue,
• place,
• stuff,
• space,
• byproduct,
• hair,
• gas,
• literature,
• view,
• fish,
• signal,
• target,
• friend,
• adulterant,
• waste,
• bedfellow,
• child,
• coating,
• entrant,
• mammal,
• surface,
• bird,
• branch,
• structure,
• molecule,
• appliance,
• advert,
• person,
• movement,
• papers,
• textile,
• chemical,
• equipment,
• extra,
• inhabitant,
• microbe,
• consequence,
• file,
• impurity,
• germ,
• sediment,
• blemish,
• compound,
• rodent,
• article,
• object,
• pet,
• smell,
• scavenger,
• debris,
• feature,
• shade,
• game,
• constituent,
• caller,
• predator,
• change,
• symptom,
• tree,
• occurrence,
• odour,
• infection,
• mineral,
• herbivore,
• parasite,
• soil,
• spam,
• treasure,
• cell,
• agent,
• sound,
• liquid,
• metal,
• fluid,
• motion,
• thing,
• vegetation,
• volatile,
• response,
• solute,
• intrusion,
• substrate,
• grub,
• piece,
• ion,
• intruder,
• atom,
• deposit,
• pesticide,
• reaction,
• behaviour,
• area,
• contaminant,
• document,
• component,
• trace,
• phenomenon,
• noise,
• feeder,
• folder,
• illumination,
• fungus,
• matter,
• protein,
• information,
• trait,
• nonsense,
• variable,
• body,
• conduct,
• pollutant,
• program,
• process,
• ingredient,
• creature,
• part,
• behavior,
• action,
• energy,
• event,
• acceleration,
• medication,
• microorganism,
• vermin,
• growth,
• situation,
• communication,
• artifact,
• emission,
• mail,
• grass,
• advertisement,
• user,
• acid,
• people,
• thought,
• comment,
• happening,
• critter,
• yeast,
• elements,