Which of the following is an example of a virus?

The cold is an example of a virus because it is caused by viral agents, primarily rhinoviruses, that infect the upper respiratory tract. Viruses are microscopic infectious agents that replicate only inside living cells of an organism. When a person contracts a cold, the virus invades the cells in the respiratory tract, leading to the common symptoms associated with the cold, such as a runny nose, sore throat, and cough.

In contrast, fungal infections are caused by fungi, which are a different type of organism entirely, while bacterial infections stem from bacteria, which are single-celled organisms that can thrive in various environments. An allergic reaction occurs due to the immune system's response to perceived threats, such as pollen or pet dander, rather than an infectious agent. Understanding these distinctions helps in recognizing the nature of different health-related issues, such as those encountered in a cosmetology setting, where hygiene and infection control are paramount.