Parasite load is a measure of the number and virulence of the parasites that a host organism harbours. Quantitative parasitology deals with measures to quantify parasite loads in samples of hosts and to make statistical comparisons of parasitism across host samples. In evolutionary biology, parasite load has important implications for sexual selection and the evolution of sex, as well as openness to experience. Infection and distribution A single parasite species usually has an aggregated distribution across host individuals, which means that most hosts harbor few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology: use of parametric statistics should be avoided. Log-transformation of data before the application of parametric test, or the use of non-parametric statistics is often recommended. However, this can give rise to further problems. Therefore, modern day quantitative parasitology is based on more advanced biostatistical methods. In vertebrates, males frequently carry higher parasite loads than females. Differences in movement patterns, habitat choice, diet, body size, and ornamentation are all thought to contribute to this sex bias observed in parasite loads. Often males have larger habitat ranges and thus are likely to encounter more parasite-dense areas than female conspecifics. Whenever sexual dimorphism is exhibited in species, the larger sex is thought to tolerate higher parasite loads. In insects, susceptibility to parasite load has been linked to genetic variation in the insect colony. In colonies of Hymenoptera (ants, bees and wasps), colonies with high genetic variation that were exposed to parasites experienced lesser parasite loads than colonies that are more genetically similar. Methods of quantifying Depending on the parasitic species in question, various methods of quantification allow scientists to measure the numbers of parasites present and determine the parasite load of an organism. Quantifying the parasite depends on what type of parasite is in question as well as where it resides in the host body. For example, intracellular parasites such as the protozoan genus Plasmodium which causes Malaria in humans, are quantified through performing a blood smear and counting the number of white blood cells infected by viewing the smear through a microscope. Other parasites residing in the blood of a host could be similarly counted on a blood smear using specific staining methods to better visualize the cells. As technology advances, more modernized methods of parasite quantification are emerging such as hand held automated cell counters,