Essential oils have been used for generations and were a foundation of treating infectious diseases prior to the discovery and manufacture of antibiotics. Plants require a defence mechanism, as do animals, to control the invasion of cells by bacteria and viruses and it is essential oils that are part of this mechanism. There are many types and the method of manufacture also affects the efficacy of the product. Most are licensed as feed additives with no direct anti-bacterial claims and therefore these products have been selected after many years investigating the best mixtures to use and when to use them. Essential oils can be applied externally as well as in water or feed and, in the right place at the right time, have proven to be a very useful component in controlling unwanted bacterial infections. Some essential oils also have a positive effect in animals with protozoal infections.