At one end of the continuum lies obligate mutualism where both host and symbiont benefit from the interaction and are dependent on it for survival.
At the other end of the continuum highly parasitic interactions can occur, where one member gains a fitness benefit at the expense of the others survival.
[3] Classically the transmission mode of the symbiont can also be important in predicting where on the mutualism-parasitism-continuum an interaction will sit.
In some systems increases in the relative contribution of horizontal transmission can drive selection for parasitism.
[5] Studies of this hypothesis have focused on host-symbiont models of plants and fungi, and also of animals and microbes.