Whenever one needs to quantify the virulence of a pathogen then this is done by using a set of laboratory animals.
LD50 (median lethal dose) is an important indicator of virulence.
LD50 is defined as the number of pathogens that are required to kill 50% of the total infected animals.
For the calculation of LD50, the animals are infected with a known number of pathogens and then a graph is plotted for the percentage of animals killed against the concentration of the pathogen used for infection.<em> The point at which the graph indicates the death of 50% animals, the corresponding concentration of inoculation of a pathogen is taken as the LD50 of that particular pathogen. </em>
<span>In order for individuals best suited for growth and reproduction in a particular environment to contribute disproportionately to the next generation natural selection must take place. Natural selection is one of the basic mechanism of evolution. This mechanism leads</span> to adaptations and adaptations contribute to survival and reproduction.
