Answer:
Maybe it was because external barriers can attack to our body and it can hurt us
Explanation:
the fact or phenomenon of light, radio waves, etc. being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density.
<span>Infection is the term used to describe the process through microorganisms that cause diseases. The invasion of a host by a pathogenic microorganism multiplies in the tissues and the reaction of the host to its presence and to its possible toxins and can be caused by bacteria, fungi, viruses, protozoa or prions.</span>
Mitosis is cell division in to daughter cells with the same chromosome number as the parent cell
