The three general types of persistent biological interactions are mutualism (both organisms benefit), parasitism (one benefits at the expense of the other), and commensalism.
Etymology: Latin for "sharing a table", from com (together) + mensa (table). The use of the word in biology began around 1870, but the use of 'commensal' as "one who eats at the same table" began in the 1400s.
The recent Scientific American article "The Ultimate Social Network" by Jennifer Ackerman discusses how babies are colonized at birth: "Each individual acquires his or her own community of commensals...from the surrounding environment. Because the womb does not normally contain bacteria, newborns begin life as sterile, singular beings. But as they pass through the birth canal, they pick up some of Mom's commensal cells, which then begin to multiply." Some of these bacteria can help the newborn to digest milk.
From Fraser's Magazine for Town and Country, volume 30 (1844), a section on desserts, fruits, sweetmeats, and liqueurs: "...we can for the life of us see no harm in talking for half-an-hour with you, gentle reader, on those commensal pleasures common to us both, which begin the earliest, and finish the latest, in this wicked world which we call ours."