Coccidiosis is a Protozoa. (a one cell organism) They live among the cells in the intestinal lining. When eggs (oocytes) are ingested each oocyte will release about a dozen sporozoites that penetrate intestinal lining. Here they increase in size becoming SCHIZONTS. They again divide and become MEROZOITES. This process continues until they become MACROGAMETOCYTES and MICROGAMETOCYTES. The MICROGAMETOCYTES fertilize MACROGAMETOCYTES that form zygotes that in turn become cysts and form oocytes.
Keep in mind all this time your pigeons intestinal lining is being torn apart allowing all kinds of bacteria to enter the blood stream.
Products to treat coccidisos are Sulmet (sulfamethazine) Baycox, and Amprolium.
Keeping your birds healthy is just as important as buying a good pigeon.