Humans have identified and isolated five different classifications of pathogens. These are Bacteria, Viruses, Fungi, Protozoa, and Worms.
Any one type of these pathogens have the potential to infect, replicate, dominate, and destroy the human body. The problem for the human eye is that all of this is occurring on a microscopic level. Similar to macro-systems, micro-systems we cannot see with the naked eye. Pathogens are existing on the microscopic level, and believe that they are real.
Because of their infectious origins, the best way to prevent pathogens is to understand how they operate.
Expert medical professionals have committed much time and energy to closely study how pathogens grow, replicate, and infect.
Pathogens are able to thrive only in living reservoirs such as human, animal, or environmental reservoirs. Any pathogen in any reservoir can be passed from one reservoir to another of similar classification, or even interchanging reservoirs.
For example, Human to Human, Human to Animal, Human to Environmental, Animal to Human, Animal to Environmental, Animal to Animal, Environmental to Human, Environmental to Animal, Environment to Environment.
Pathogens can cause a range of symptoms in humans and animals. Depending on the pathogen type along with individual immune response, symptoms can be mild such as rash or cough to severe such as cardiovascular or respiratory issues.
The modes of transmission are via direct or indirect contact. Direct contact is that of physical contact transmission whether directly (skin to skin) or a direct extension ( saliva, or other bodily fluid to another). Infection by indirect contact is that of a reservoir pathogen passed to a host by either airborne transmission, vehicle transmission (water, food, other biological environments), or vector transmission (mosquitos, ticks, fleas).
Direct contact is usually between human to human or human to animal, whereas indirect contact can be transmitted by a vector by means of contact with an infected human to a non-infected human, an infected animal to a non-infected animal, or an infected animal/environment to a non-infected human, etc.
Pathogen existence and the spread of contagion is a very real threat if the proper precautions are not taken into consideration. Three out of four new or emerging pathogens in humans come from animal origins. Remember animal to human contact can be direct, or indirect even in the form of food.
keeping a balanced nutritional lifestyle to maintain strong immunity
cleaning and disinfecting if exposure is 'high-risk'
staying aware of potential droplet spread such as coughing, sneezing, and even open-wound exposure
remaining cautious of water and food sources which pathogens might use to survive until a more suitable host is discovered, allowing the pathogen to infect and reproduce.
Source: Immunobiology: The Immune System in Health and Disease. 5th edition.
Janeway CA Jr, Travers P, Walport M, et al.
New York: Garland Science; 2001.