There are numerous circumstances that give pathogens the chance to harm people.
The first occurs when an infection has already weakened the immune system. In situations like this, other pathogens may infect the host opportunistically and cause an infection. If the human host does not maintain proper personal and environmental hygiene, that is another condition. Many pathogens thrive in unclean environments, so by failing to maintain proper environmental hygiene, a person increases his risk of contracting a pathogen.
Finally, since so many pathogens are transferred by vectors, humans are more likely to contract the diseases that these vectors aid in spreading if they do not take the necessary precautions to stop their activity.
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Answer: d. A secretary smokes a cigarette in a crowded break room.
Explanation:
An external costs can be define as the activity which is likely to disturb the nature or cause harm to a living being. d. is the correct option because of the fact that if the secretary smokes a cigarette the smoke likely to disturb the crowd in the break room as some people may feel suffocated because of the smoke due to obstruction of natural air in the nostrils.