If pathogenic microorganisms are all around us, including in the air, on surfaces, and even on our own skin, explain why we don't all develop new infectious diseases every day

Question

What will be an ideal response?

Answer 1

Answer: Three conditions must be present at the same time to develop disease from exposure to a microorganism.
1) The agent must be able to transmit the pathogen. In other words, there must be contact—through broken skin, inhalation, and so forth—between the pathogen and the interior of the human body. Skin, for example, is the body's first defense: pathogens can gain entry to the body via cut or burned skin, but are not able to gain entry in a person with intact skin.
2) The host must be susceptible; that is, vulnerable in some way to the infection. Most obviously, the host's body temperature, chemistry, and cellular make-up must be hospitable to the pathogen involved. This explains why, for example, a certain type of influenza that is pathogenic in dogs may not be pathogenic in humans. Moreover, within any given community, some people have a strong immune system, whereas others—including the very young, the very old, pregnant women, and people who are ill—have a compromised immune system unable to destroy the pathogen before it replicates to cause disease.
3) The environment must allow for the pathogen's survival. Factors such as temperature, light, and moisture must be hospitable to the pathogen. Pathogenic microorganisms in contaminated foods cannot reproduce in your freezer, for example, but can replicate quickly in the same food left out on your kitchen counter.