infestation       
noun [ C or U ] uk/ˌɪn.fesˈteɪ.ʃən/ us/ˌɪn.fesˈteɪ.ʃən/
 侵扰,大批出没
a large number of animals and insects that carry disease, that are present where they are not wanted
  a flea infestation 一群跳蚤侵扰 
 an infestation of cockroaches/head lice 一群蟑螂/头虱侵扰 
 See: infest
 - More examples
 - This disease is caused by infestations of mites in the skin.
 - Having children's hair cut extremely short may aid in the detection of lice but will not prevent infestation.
 - Millions of trees in the West have died from insect infestations.