What is the life cycle of a pathogen? Pathogens want to survive and reproduce, and so they usually do it at the expense of a host who provides an appropriate environment in assisting the survival of the pathogen. For pathogens to to survive and reproduce, there are a number of steps that must be taken.

• Portal of entry: Invade the host

• Exploit: Use a nutrient-rich area of the host

• Defence mechanisms: Avoiding these

• Replicate: Make more copies

• Portal of exit: Be able to exit and transmit to new hosts (transmission)

This is termed the pathogen's life cycle, which is crucial in identifying suitable strategies for controlling a pathogen which is usually done by intercepting one of these steps in the cycle.

How do pathogens invade a host? Pathogens may invade a host through various means; the susceptible host has various portals of entry, which can be things like mucous membranes, skin, wounds, eyes and ears. Mucous membranes are found in the human respiratory, gastrointestinal and reproductive tracts and are surface membranes which are moistened by slimy, viscous mucous. Mucous is produced by epithelial cells lining the membranes. In both plants and humans, the outer layer of tissue is called the epidermis which usually provides a physical barrier to most pathogens. However, the external openings of systems such as the respiratory, digestive, excretory and reproductive systems provide optimal points of entry for many pathogens. If the skin is wounded, a pathogen has the ability to penetrate the barrier and exploit the wound for entry.

What is the impact on the host? Following entry of a pathogen, many of them attach to host cells to which they multiply either in between host cells or within body fluids, or they actually enter the host cells and replicate there. The impact of the invasion varies dependent on the pathogen, but people may present with varying degrees of signs and symptoms. Symptoms are described as being subjective feelings or experiences of a patient which are not diagnosable, such as pain and nausea. Signs are more objective and measurable, including vital signs such as temperature, heart rate and breathing rate. How do pathogens exit a host? Pathogens leave a host through numerous portals of exit, which allow for transmission to occur. A number of examples exist, including;

• A bite, blood feed or saliva

• Digestion (the elimination of waste products)

• Respiratory system (through coughing and sneezing, aka protective reflexes)

• Blood contact (sharing of unclean needles etc.)

• Reproductive systems (i.e. a queen bee may pass a virus to worker bees)