HTT has been shown to occur between species and across continents in both plants and animals (Ivancevic et al. 2013), though some TEs have been shown to more successfully colonize the genomes of certain species over others. Both spatial and taxonomic proximity of species has been proposed to favor HTTs in plants and animals. It is unknown how the density of a population may affect the rate of HTT events within a population, but close proximity due to parasitism and cross contamination due to crowding have been proposed to favor HTT in both plants and animals. Successful transfer of a transposable element requires delivery of DNA from donor to host cell (and to the germ line for multi-cellular organisms), followed by integration into the recipient host genome. Though the actual mechanism for the transportation of TEs from donor cells to host cells is unknown, it is established that naked DNA and RNA can circulate in bodily fluid. Many proposed vectors include arthropods, viruses, freshwater snails (Ivancevic et al. 2013), endosymbiotic bacteria, and intracellular parasitic bacteria. In some cases, even TEs facilitate transport for other TEs.