Disease-causing organisms like viruses, bacteria, fungi and parasites are the leading cause of death worldwide according to MedlinePlus. These organisms---called pathogens---can tear down cell structures and weaken the body's ability to function normally. Pathogens come in different forms and strengths. though the condition of the host body plays a role in how virulent a particular pathogen can be.
A pathogen is defined as any organism that's capable of causing disease or infection within the body. Viruses, bacteria, fungi and parasites are all classified as pathogenic organisms. Pathogens represent the first link in the chain to infection, according to the University of Arizona. In order to become a threat, an organism must attach itself to a host or body. A mode of transmission and a certain degree of susceptibility in the host are also needed for pathogen organisms to take root.
A virus pathogen is an aggressive organism that can quickly spread throughout the body. Its physical characteristics enable it to initiate a disease or infection-causing process, according to the University of Arizona. Viruses are classified as intracellular parasites, meaning they survive by feeding off of a host cell's metabolism processes. Once inside the cell, virus DNA and RNA molecules attempt to take over the host cell's genetic functions. The virus then spreads as newly formed cells incorporate the virus' genetic materials.
Bacteria provide another illustration of how pathogens take root in the body. Bacteria are single-celled organisms that can live inside or outside a host environment, according to the University of Arizona. Those that require a host live outside of individual cell structures, but still derive a certain amount of nutrients from the host. Other types can make their way inside of cell structures and draw enzyme and nutrient materials from the inside of the cell. Bacteria spread through binary fission where each parent cell manufactures two identical daughter cells.
Fungi pathogens appear as unicellular or multicelluar organisms with self-sufficient cell structures, according to the University of Arizona. Their cells sustain themselves through DNA-RNA protein production processes much like the cells in the human body. Like bacteria, they can survive with or without a host body. Fungi organisms are only a threat when a host's immune system is weakened. Like bacteria, they exist outside of host cell structures and typically attach themselves to the surface of the host body.
Parasites begin their journey as unicellular organisms that change into different forms as they move through their life cycle. Each stage in the cycle is characterised by a certain appearance, chemical make-up and environment, according to the University of Arizona. Parasites can survive with or without a host body depending on what stage of the cycle they're in. Those that do attach to a host body survive by feeding off of host cells, literally eating away at cell structures.