This project is centered around a unique study system funded by VR to investigating the evolution and ecology of avian malaria parasites in a wild bird population, located in southern Sweden. The system has been monitored for more than 28 years and the prevalence of a specific locally transmitted blood parasite has doubled from 45% in the mid -90s to 90% of all the adult birds in the population being infected over the last five years. By using historically collected samples from this well monitored and sampled host population, together with newly developed amplification methods (SWGA) which allows us to sequence the genomes of the parasite we have set the following general goal. • General goal: In a natural host-population understand the underlying population genomics, short-term evolution and epidemiology of a malaria like blood-parasite that have doubled in prevalence, from 40% in 1996 to >85% in 2021. In order to achieve this general goal, we have the following specific aims for this part of the project. o Specific aim 1) What is the underlying level of genetic variation that occur at the genomic level in the parasite population. o Specific aim 2) Investigate if new parasite genotypes has been introduced and increased in the host population or if there have been a fixed set of genotypes that have changed in frequencies over the years? o Specific aim 3) How do the genetic host-pathogen interactions changes across a quarter of a century as a result of genetic changes in the parasite population is across temporal and spatial scales. o Specific aim 4) What is the degree of vertical transmission of the parasite in the population, and if vertical transmission occurs do the female and male contribute differently to the spread of the disease? Understanding the genetic heterogeneity of pathogens across space and time is crucial in order to understand co-evolution between hosts and pathogen and to understand the dynamics of how new pathogen strains spread in host populations. It is further of importance when trying to estimate fitness consequences for the host, as a genetically homogenous pathogen population might affect the host population fundamentally different compared to facing a pathogen population with multiple variants that fluctuates over time and space. In this study we have a unique data/sample set, allowing us to investigate the above questions at fine detail.