Is a leading cause of death among the sick and elderly.

The correct answer is pneumonia.

Explanation:

The elderly suffering from pneumonia makes them susceptible to various other issues. Due to its rapid occurrence and tendency to disperse to other sections of the body, pneumonia can be lethal. Even a mild case of pneumonia results in an extreme blow to the immune system that can cause an underlying condition like a heart condition into a life-threatening problem.  

As older individuals generally possess these additional issues, any kind of pneumonia can possess dangerous consequences.  

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link:

interphase

the period of cell growth when the cell synthesizes new molecules and organelles. the cell is not dividing and it constitutes the majority of the time spent in the cell cycle

prophase

changes occur in the nucleus and cytoplasm. the chromatin condenses to form structures (sister chromatids) visible with a light microscope and the mitotic spindle begins to form, but the nucleus is still in tact

prometaphase

the nuclear envelope fragments and the spindle microtubules attach to the kinetochores of the sister chromatids

metaphase

all the cell's duplicated chromosomes are lined up at an imaginary plane equidistant between the poles of the mitotic spindle

anaphase

begins when sister chromatids separate from each other and ending when a complete set of daughter chromosomes arrives at each of the two poles of the cell

telophase

the daughter nuclei form at the two poles of a cell

cytokinesis

the division of a cytoplasm to form two separate daughter cells. usually occurs during telophase during mitosis. this and mitosis make up to mitotic (m) phase of the cell cycle