The Prime Minister used the emotive term as he discussed measures being taken to deal with chaotic scenes at Calais, where large numbers of migrants living in a tent city have attempted to enter the Channel Tunnel.
Speaking on a visit to Vietnam, Mr Cameron told ITV News that the problem had become worse in recent months because "you have got a swarm of people coming across the Mediterranean, seeking a better life, wanting to come to Britain".
Last October, the PM said ministers should "choose our language carefully" on immigration, after Defence Secretary Michael Fallon suggested some towns were being "swamped" by migrant workers.
Downing Street defended his comments today, telling reporters: "It was clear that he was talking about people who are seeking a better life. It was clearly talking about people."