bio

Symbiosis

A close, long-term living-together of two different species. Symbiosis does not mean both benefit — it is a spectrum: mutualism (both gain), commensalism (one gains, the other is unaffected) and parasitism (one gains at the other's expense). It can happen outside the body (ectosymbiosis) or inside the host's own cells (endosymbiosis). The boundaries between these forms are often blurred and can slide: a mutualist can tip into a parasite when conditions change.