Here is your short paragraph on the evolution of parasites!
Parasites are organisms belonging to small isolated groups.
The degree of parasitism exhibited in the form of host-parasite relationship, which we observe today, have probably evolved gradually through ages.
It is difficult to explain step by step the details of the process of evolution of parasites from simple, temporary, harmless host-parasite relationship to a more developed, intricate, specialized and pathogenic parasitic forms.
It is generally believed that parasitism have evolved from free living forms but there is no recorded instance exhibiting sudden change from free living to parasitic mode of life. Naturally, parasitism has evolved gradually which is evidenced by so many gradations of parasitic existence.
In the earliest stage of parasitic evolution the host acted simply as a vehicle of transportation. Later on the parasite became dependent upon their host for obtaining food. In this way ectoparasites evolved.
The endoparasites too, are believed to have evolved from free living forms, which reached accidently into the host body. Whether endoparasitism evolved directly or from ectoparasitism or commensalism or symbiosis is unknown, however in any case endoparasitism brought about enormous changes for parasitic existence to meet the demand for proper adjustment with the respective host through the process of mutation and natural selection. Further, evolution led to the development of multi-host parasitic species as their survival required intermediate host.