[6][7] Babesia comprises more than 100 species of tick-borne parasites that infect erythrocytes (red blood cells) in many vertebrate hosts.

[8] Babesia species infect livestock worldwide, wild and domestic vertebrate animals, and occasionally humans, where they cause the disease babesiosis.

[2] The avian Babesia species are characterized as having ring and amoeboid forms, and fan-shaped or cruciform (cross-shaped) tetrad schizonts.

In 1888, Victor Babeș first identified the causative agent in Romania and believed it to be due to the bacterium he named Haematococcus bovis.

[7][12] In 1893, Americans Theobald Smith and Fred Kilborne identified the parasite as the cause of Texas cattle fever, the same disease described by Babeș.

They also identified the tick as the transmitting agent, a discovery which first introduced the concept of arthropods functioning as disease vectors.

[15] Too lethal an infection results in the host's death and the parasite is unable to spread, which is a loss from an evolutionary standpoint.

[citation needed] The lifecycle of B. microti, which is typical of parasites in the genus, requires a biological stage in a rodent or deer host.

[17] Trophozoite and merozoite growth ruptures the host erythrocyte, leading to the release of vermicules, the infectious parasitic bodies, which rapidly spread the protozoa throughout the blood.

[19] Warm, wet weather increases the intensity of infestation—the population is able to thrive due to the relatively fluid environment, making water and nutrients more accessible.

The tick, if not removed, stays attached for three to four days, with longer periods of feeding associated with a higher probability of acquiring the parasite.

Their reservoirs are theorized to be the white-footed mouse (Peromyscus leucopus), voles from the Microtus genus, and the white-tailed deer (Odocoileus virginianus).

As of 2003, the Centers for Disease Control and Prevention (CDC) acknowledged more than 40 cases of babesiosis contracted from transfusions of packed red blood cells (PRBC), as well as two infections documented from organ transplantation.

As symptoms may not appear, many women may not be aware they are infected during pregnancy, so a measurement of congenital transmission rate is not known at this time.

[25] Of the species to infect humans, B. microti is most common in the Americas, whereas B. divergens is the predominant strain found in Europe.

[26] Ixodidae, the tick vectors of B. microti, also transmit the better-known Borrelia burgdorferi, the causative agent of Lyme disease.

B. poelea was described from brown boobies (Sula leucogaster) on Sand Island, Johnston Atoll, Central Pacific.

[28] B. poelea was reported from a masked booby (Sula dactylatra melanops) from Desnoeufs Island, Amirantes, Seychelles.

About 40 cases of human babesiosis, caused by intraerythrocytic protozoans (protozoa inside red blood cells) of the genus Babesia, were reported in Europe.

In other cases, symptoms are characterized by irregular fevers, chills, headaches, general lethargy, pain, and malaise.

[9] In severe cases, effects of parasitic multiplication, symptoms such as hemolytic anemia, jaundice, shortness of breath, and hemoglobinuria have been documented.

If left untreated, B. divergens infections can develop into shock-like symptoms with pulmonary edema and kidney failure.

[11] B. peircei infections can cause mild anemia, leukocytosis, and impairment of hepatic function in African penguins.

Trophozoite and merozoite growth ruptures the host erythrocyte, leading to the release of vermicules, the infectious parasitic bodies, which rapidly spread the protozoa throughout the blood.

[9] It is important to pay attention to particular morphologies of Babesia in blood smears, because of its great similarity to the malarial parasite Plasmodium falciparum.

The few distinguishing factors for Babesia include protozoa with varying shapes and sizes, the potential to contain vacuoles, and the lack of pigment production.

To prevent transmission of Babesia and other tickborne pathogens, all birds with visible ectoparasites are treated with pesticide powder (carbaryl 50 g/kg) upon admission, and the facilities are thoroughly cleaned on a daily basis.

[7] The disease was eradicated from the United States by 1943, except for a permanent quarantine area along the Texas/Mexico border, where cattle fever ticks are still found.

[7] Complete eradication through vector control would be a long-term project, which would significantly reduce the prevalence of both babesiosis and Lyme disease, but as public health departments are often short on funding, preventive measures are more recommended.

[citation needed] Due to the relatively low prevalence of the human disease and the presence of several reservoirs, babesiosis has not been a candidate for vaccines.