With respect to the compatibility filter, a role for unsuitability should not be discounted and could be manifested in several ways, such as a lack of receptors needed for efficient viral entry into cells (Parrish et al., 2008) , lack of appropriate structures for parasite attachment (Tompkins and Clayton, 1999) , or by a general failure to provide the biochemical environment needed for the parasite to survive (Sullivan and Richards, 1981) .