Modifiers matching intruder:

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

Simple categories matching harmful:

• chemical,
• defect,
• state,
• substance,
• volatile,
• factor,
• metal,
• fauna,
• condition,
• intermediate,
• herb,
• organism,
• email,
• material,
• inhalant,
• addiction,
• compound,
• sediment,
• trait,
• emotion,
• fumes,
• particle,
• residue,
• germ,
• program,
• result,
• pest,
• software,
• microorganism,
• stain,
• pathogen,
• power,
• bacteria,
• contaminant,
• stress,
• sector,
• information,
• stuff,
• act,
• property,
• exposure,
• gas,
• allergen,
• solute,
• drug,
• flavoring,
• cell,
• energy,
• fish,
• sweetener,
• bug,
• salt,
• constituent,
• tax,
• ingredient,
• alkaloid,
• emission,
• frequency,
• halogen,
• arachnid,
• beverage,
• change,
• pollutant,
• filler,
• item,
• radiation,
• solvent,
• impurity,
• process,
• notion,
• influence,
• elements,
• effect,
• stimulus,
• quality,
• creature,
• drinking,
• fumigant,
• invertebrate,
• treatment,
• aggression,
• liquid,
• fat,
• tool,
• industry,
• nutrient,
• contamination,
• antigen,
• parasite,
• waste,
• consequence,
• cause,
• sport,
• code,
• complication,
• therapy,
• procedure,
• commodity,
• irradiation,
• phenomenon,
• practice,
• fluid,
• product,
• fraction,
• medication,
• content,
• intruder,
• monomer,
• residual,
• microbe,
• oxidant,
• group,
• ion,
• utterance,
• problem,
• action,
• animal,
• fuel,
• yeast,
• activity,
• choice,
• disorder,
• food,
• deposit,
• element,
• virus,
• deeds,
• experience,
• preservative,
• place,
• agent,
• reactant,
• crime,
• illness,
• mineral,
• danger,
• circumstances,
• object,
• thing,
• abnormality,
• service,
• idea,
• radical,
• plant,
• pathology,
• behaviour,
• article,
• work,
• gene,
• matter,
• insect,
• force,
• event,
• molecule,
• diuretic,
• subsidy,
• pesticide,
• behavior,
• occurrence,
• toxin,
• infection,
• light,
• adulterant,
• predator,
• disease,
• occupation,
• weather,
• situation,
• entity,
• environment,
• fiber,
• test,
• caterpillar,
• component,
• habit,
• atmosphere,
• curio,
• conduct,
• weeds,
• intervention,
• aerosol,
• stimulant,
• symptom,