March 15, 2021 6:10 am Coccidiosis occurs in calves from the approximate age of three weeks onwards. It arises in young calves who have an underdeveloped immune system and are being housed in dirty, unhygienic accommodation. Infected calves excrete large amounts of oocytes (eggs) which, in turn, contaminate the environment and lead to the rapid spread of the disease. Coccidiosis is highly contagious and oocytes can survive in the environment if the housing conditions are favourable. According to Teagasc, cattle can develop immunity to the disease. However, cattle experiencing subclinical infection will show very few symptoms and will recover with time; but thrive will be affected.