Individual constitution plays a role and those with a weaker system must work harder to keep their immune system healthy. The pathogens that cause infections are bacteria, fungi, animal parasites and viruses. Bacteria, fungi and parasites generally invade body tissue, steal nutrients from healthy cells and release toxins. Viruses are really sub-life forms that survive only by invading living cells and multiplying inside them. Understandably, the parts of the body that are most accessible to pathogens are those most vulnerable to infection.
Pathogens readily infect the eyes, ears, mouth, genitals, and the skin itself.
Skin infections are extremely common and can be caused by any type of pathogen. Other pathogens can enter the body through broken skin and infect the blood stream. Any type of pathogen can invade the gastrointestinal tract, usually through contaminated food or water. Inhaling or ingesting viruses causes most respiratory infections, including common colds, flu and mild forms of pneumonia. Severe cases of pneumonia are usually bacterial, as are whooping cough and strep throat.