What general feature is necessary to consider an organism a microbe?

to be unicellular.

Explanation:

A microbe, is a very small (microscopic) unicellular organism that can only be seen with a microscope. In some cases these organisms form colonies (aggregates of unicellular organisms). Microbes are part of two of the the three domains of life and are highly diverse and inhabit almost every habitat in the planet earth.

it would lead to upper respiratory infections and pneumonia.

the correct answer is: b. pantala flavescens, because these organisms would have a high level of genetic variation due to sexual reproduction.  

because involves two organisms (parents) whose genetic material combines, sexual reproduction leads to genetic variation in new generations of offspring. the offspring is an unique organism, with new genome, more protected with the higher ability to adapted. this process is fundamental to evolution.