Symbiosis is a co-evolved intimate relationship between members of two different species.

There are two types of symbiotic relationship — parasitism and mutualism.

A parasite benefits in terms of energy or nutrients, whereas its host is harmed by the loss of these resources.

Parasites often have limited metabolism and cannot survive out of contact with a host.

Parasites can be transmitted to new hosts using

• direct contact
• resistant stages
• vectors

Some parasitic life cycles involve intermediate (secondary) hosts to allow them to complete their life cycle.

In mutualism, both mutualistic partner species benefit in an interdependent relationship.