The host parasite relationship is completely dependent on the interaction between two species. The interrelationship between the parasite and host populations incorporates with the infection level. Obligate parasites are mostly pathogenic bacterias, which are associates with their hosts. Examples of such pathogenic bacterias are staphylococci and streptococci. Parasite host can survive and increase the population outside the body of the host and however, it infects the host source. Within the host tissues, these organisms set up the infections and start spreading throughout the body. Other bacterias like Glanders bacillus, Gonococci, Meningococci, and pneumococci are closely adapted parasites. These bacterias are capable of multiple outside the body of the host that is only under the artificial conditions of the laboratory. All these bacterias have complete metabolic capabilities and cell structures.
The rickettsiae and viruses show a greater degree of dependence on the host cells. The rickettsiae is a kind of microorganisms that have cell structures like bacteria. These organisms exhibit a small degree of metabolic activity outside the cells, but they cannot grow in absence of host cells. Viruses in the form of parasitism, do not have a conventional cell structure. And it contains only DNA or RNA as nucleic acid wrapped with the protective coat of protein. Viruses are termed obligatory intracellular parasites, which are capable of multiplying within the host cells and they do not have independent metabolic activities for their own. The genetic information present in the virus will synthesis certain enzymes to enters the host cell to perform chemical processes and directs them towards the synthesis of new viruses. This shows the host parasite relationship in medical microbiology.
The condition of obligate parasitism varies with the degree of parasites and hosts. That is the parasite host is closely adapted to specific species as its host. About 90% of microorganisms that adopt to the plant host cannot adopt the animals. Likewise, the microorganisms which can survive in the animal host rarely accept the plant host. The rate of infection depends upon the relationship between parasite and host. The taxonomic relationships include the host variety it may depend on vertebrates and invertebrates.
The negative survival value of the parasite often shows the consistent fatal disease in host species. This is termed a negative survival parasite as it can eliminate quickly from the host. If the host cells start adopting the parasites, which causes infection in the host, the infection rate will become less and start to tolerate it. The best example for reducing the severity of the disease can be absorbed from the spirochete, which causes syphilis in human beings. The severity of syphilis is more in the 16th century. But it started to reduce gradually, once the host gets accepted to the disease.
On the other hand, the ecological studies of parasitism show that it is incorrect to assume host-parasite relationships as it gets evolve towards reduced antagonism because the disease-producing pathogens are also started evolving according to environmental changes. This state is known as ameliorates.
The severity of the disease produced in host species may be milder or high depends on definitive hosts. In certain cases, the pathogen or microorganism which attacks the host will keep the host cells undisturbed under natural conditions. This condition is true for many of the disease-producing microorganisms in human beings. On the other hand, when the same infection transferred to the secondary hosts may cause severe fatal disease. For example, rabies is a fatal disease for all animal hosts, but the same rabies infection-causing virus will persist for long periods in bats and remains an asymptomatic infection.
The specificity of pathogenic microorganisms mainly depends on their hosts. The microbial characters vary depending on the host resistance. The microorganism cause disease and the severity of the disease mainly depend on the host reaction, resistance, immunity of the host reaction regard to its effect on the microorganism. The effects of the host may remain constant or vary accordingly. For example, virulence is an infective agent that determined experimentally by inoculating groups of hosts. The graded doses of the agent determine by interpolation. The host individuals inoculate with the dose of infection. This dose is called ED50 or median effective dose. It is the direct way of building resistance related to the inverse fashion of virulence.
An organism will experience less virulent for infection while providing a higher dose of ED50. If the higher dose of ED50 is tested against the resistance of a particular host, these microorganisms are known as virulence. The resistance of different host species is expressed as an n-fold, it gets increases or decreases in ED50 over the normal host species. This method of testing is possible for both the virulence and resistance of the host cells. The member of host and microorganism populations occupy a central position, it depends on the exceptional individuals appears at both extremes. According to the host resistance, the varied incidence of disease in a host population will increase to a statistically constant dose depends on the infectious agent. In most cases, the dosage varies greatly from one host to another depends on its circumstances related to an infectious agent. Every individual has different host resistance to infection. However, the generation of more resistance against in host cell is due to encountering more infectious agents. This results in the innate factor in the individual host organism. If the whole population is exploring the particular infection for the first time, then the host resistance of the individual remains the same.