There are several pathogen types that can cause disease include many groups of single-celled microorganisms and larger multicellular parasites. Viruses, bacteria, some yeasts, and protozoan parasites are examples of single- celled pathogens. Fungi and helminths (parasitic worms) are the major multi-cellular pathogens. These pathogens come from very different parts of the biological kingdom and vary considerably in many aspects. Pathogens differ enormously in their size. They also have very different lifestyles and cause disease in a variety of ways like bellow:
Poliovirus (Viruses)
Size: 20–400nm
Habitat: Intracellular: pharynx, intestine, nervous system
Mode of multiplication: Intracellular synthesis of viral components
Multiplication rate (doubling time): <1 hour
Poxvirus (Viruses)
Size: 20–400nm
Habitat: Intracellular: upper respiratory tract, lymph nodes, skin
Mode of multiplication: Intracellular synthesis of viral components
Multiplication rate (doubling time): <1 hour
Streptococcus pyogenes (Bacteria)
Size: 1–5µm
Habitat: Extracellular: pharynx
Mode of multiplication: Cell fission
Multiplication rate (doubling time): 3 hours
Mycobacterium leprae (Bacteria)
Size: 1–5µm
Habitat: Intracellular: macrophages, endothelial cells, Schwann cells
Mode of multiplication: Cell fission
Multiplication rate (doubling time): 2 weeks
Candida albicans (Fungi)
Size: 2–20µm
Habitat: Extracellular: mucosal surfaces
Mode of multiplication: Asexual budding
Multiplication rate (doubling time): Hours
Histoplasma capsulatum (Fungi)
Size: 2–20µm
Habitat: Intracellular: macrophages
Mode of multiplication: Asexual budding
Multiplication rate (doubling time): Hours
Trypanosomes (Protozoan parasites)
Size: 1–50mm
Habitat: Extracellular: bloodstream
Mode of multiplication: Binary fission
Multiplication rate (doubling time): 6.5 hours
Plasmodium (Protozoan parasites)
Size: 1–50mm
Habitat: Intracellular: red blood cells, hepatocytes
Mode of multiplication: Asexually in hepatocytes (cell fission)
Multiplication rate (doubling time): 8 hours
Ascaris lumbricoides (Metazoan parasites worms)
Size: 3mm to 7m
Habitat: Intestine
Mode of multiplication: Lays eggs
Multiplication rate (doubling time): 200000 eggs/day
Taenia solium tapeworm (Metazoan parasites worms)
Size: 3mm to 7m
Habitat: Gut
Mode of multiplication: Releases body segments containing eggs
Multiplication rate (doubling time): 800000 eggs/day
Size of pathogens
One feature of the range of pathogenic organisms listed above is the enormous variation in size. Viruses are the smallest infectious organisms, being 20–400 nm in size. At the other end of the scale some parasitic worms, such as the tapeworm, can be up to 7 m (20 ft) in length. This represents a difference in scale of a factor of 10e9 . To put that into some sort of perspective, if a virus were the size of a tennis ball, a fully developed tape- worm would reach from London to Los Angeles. It does not stretch the imagination too far to appreciate that the problems posed to the immune system by these two organisms would require very different solutions.
Stages of disease production by pathogens
Size is not the only way in which infectious organisms vary. They also vary enormously with respect to how they enter and live within the body and actually cause disease. Infection and disease production by pathogenic organisms can be divided into four stages:
1. Invasion.
2. Multiplication.
3. Spread.
4. Production of disease (pathogenesis).
Although infection usually involves all of these steps, there are many exceptions in terms of both the steps involved and their order. Some pathogens do not spread significantly or even technically gain entry to the body. Organisms may replicate locally before spreading or may spread through the body before beginning significant replication. Pathogens show considerable variation at each of these stages of infection, as will be described below.
