PARASITE INFECTIVITY 



habitually lives in a certain host species is found in a 

 host that is very seldom infected, that host is spoken of 

 as an accidental or casual host. A host may become in- 

 fected but throw off the infection after a short time, in 

 which case it is known as a provisional or transitory host ; 

 or it may serve as a host for a short stage in the life-cycle 

 of a parasite, thus becoming a temporary host. 



An infection may be acute, malignant, fulminating, 

 chronic or benign, but the evidence does not indicate that 

 the susceptibility of the host to an infection has any bear- 

 ing on the character of the infection induced. That is 

 to say, a host may be more susceptible to infection, and 

 probably usually is, by a species of parasite that never 

 calls forth symptoms than by a pathogenic or lethal 

 species. 



2. PARASITE INFECTIVITY 



If a host is easily parasitized by a species, the parasite 

 is said to be highly infective. How much its infectivity 

 is due to the host and how much to the parasite it is 

 impossible to say. Several of the terms noted above with 

 respect to hosts are also commonly used to designate dif- 

 ferent types of parasites. Thus, we speak of natural 

 parasites, accidental parasites, and provisional, transitory 

 or temporary parasites. Parasites are also classified ac- 

 cording to the necessity of existence within a certain 

 host as facultative, when this is not required, and obli- 

 gate, when the parasite is unable to live in any other host. 

 The invasive powers of a parasite are indicated by such 

 terms as virulent or aggressive and the degree of infec- 

 tivity with respect to the effects on the host as patho- 

 genic, sublethal, and lethal. 



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