The process of infection can be broken down into stages, each of which can be blocked by different defense mechanisms. In the first stage, a new host is exposed to infectious particles shed by an infected individual. The number, route, mode of transmission, and stability of an infectious agent outside the host determines its infectivity. Some pathogens, such as anthrax, are spread by spores that are highly resistant to heat and drying, while others, such as the human immunodeficiency virus (HIV), are spread only by the exchange of bodily fluids or tissues because they are unable to survive as infectious agents outside the body.
<u>Cell is the most basic unit of matter that can carry out all the processes of life however not all cells are alike.</u>The different structures found within the cell is called the organelles and are assigned to perform specific function in the specific cell. It is also said that organelles maintain the life of the cell. The organelles of a single cell like the organs of a multicellular organism is very essential for the survival of the body. <em>The presence of organelles in both unicellular and multi-cellular organisms are very essential in orde to maintain life and carry on with its individual functions.</em>
