Explain two possible reasons why a person who looks well could still be a source of infection microorganism?

- many pathogens exhibit a latency (asymptomatic) period of incubation  

- usually, pathogens need to replicate many times in order to be transmitted between hosts (i.e., they need to increase the pathogenic load)

Explanation:

The latency stage refers to the period between being exposed to a particular pathogenic microorganism (for example, a virus) and has symptoms associated with the disease caused by such pathogen. Generally, this incubation stage is asymptomatic, thereby a person will look healthy during this period. Moreover, the pathogenic load (usually known as the viral load in viral infections) can be defined as the number of pathogenic microorganisms in an infected person's blood. During the first stages of infection, the pathogenic load usually is low, whereby a person can also look healthy during this period.

the answer is:

carbon dioxide

not sure if this is what you'r looking for eubacteria and archaebacteria