R₀ and the vector-pathogen-host triangle within the environment. Interactions amongst all three organisms shape the adaptations of LB spirochetes for vectorial transmission. At the heart of the triangle is R₀, the reproductive number for LB spirochetes, which is a measure of its fitness in a given environment, where N/H is the ratio of ticks to vertebrate hosts; f is the combined probability of a tick contacting and feeding successfully on a host; β_V-T is the transmission coefficient for LB spirochetes from an infected vertebrate to a tick; β_T-T is the transstadial transmission coefficient through the molt from one life stage to the next one; β_T-V is the transmission coefficient for LB spirochetes from an infected tick to a vertebrate; p is the tick’s daily survival probability; n is the extrinsic incubation period; F is the vector’s fecundity; r is the host’s daily recovery rate; and h is the host’s daily mortality rate. See ^Appendix for further explanation.