**2.1 Cycle and routes of infection**

The epidemiologic triangle (triad model) explains that infectious diseases are produced from the interaction of a causative agent, susceptible animal (host), and environmental surrounding. More attentionally, transmission of the causative agent results when the agent leaves its animal (*host* or *reservoir*) through a way of escape (portal of exit), is produced by some *means of transmission*, and enters through a specific entrance (portal *of entry*) to infect a *susceptible animal*. This cycle is sometimes called the chain of infection.

When speaking of infectious diseases, *exposure* assumes a tailored meaning, namely, encountering the infectious agent in a fashion that allows for agent transmission. Therefore, knowledge of transmission mechanisms is vital for understanding the epidemiology of infectious diseases.

This chapter is focused on major bacterial diseases that can cause significant loss or concern for dairy cattle, beef rearing, and fattening enterprises, as well as practical steps that can be taken to prevent the occurrence of such diseases and regulatory control measures that should be taken on a specific disease.
