These duplication, fusion, and fission events form multiple gene copies and fragments, which are able to accumulate mutations.

With a higher range of outer membrane proteins, the parasite can evade the immune system of the host more effectively and establish persistent infection.

[5] The most pronounced evidence of evolution in the genome size of Erhlichia is the presence of tandem repeats,[3] which vary highly among individuals and species.

Over time, individuals may expand or contract parts of their genes and alleles, which adds genetic variation and may sometimes affect phenotype.

[3] Ehrlichia and its closely related genus Anaplasma show extreme diversity in the structure and content of their genomes.

[6] The evolutionary changes in the outer membrane proteins have led to the emergence of new strains that can infect a larger variety of hosts.

Heartwater, caused by E. ruminantium, is a prevalent tick-borne disease of livestock in Africa and the Caribbean, but also threatens the American mainland.

The more outer-membrane protein genes that can be expressed, the higher the chance the organism can avoid being recognized by the host's immune system.

[9] Amblyomma americanum ticks spread E. chaffeensis and E. ewingii bacterial infection in the Eastern and Southeastern United States, while A. phagocytophilum is spread by the Ixodes scapularis tick in the Upper Midwest; 1,518 cases of E. chaffeensis were recorded in southeastern, south-central and mid-Atlantic areas of the country in 2013.

[10] During 2008–2012, 4,613 cases of E. chaffeensis infections were reported through the National Notifiable Diseases Surveillance System (NNDSS).

[11] In Minnesota and Wisconsin, four people reported symptoms that are associated with ehrlichiosis, and upon further research, neither of these cases was found to be E. chaffeensis or E. ewingii, but instead it was revealed as a new species, similar in genetic makeup to E.