Final answer:
Chickenpox, caused by the varicella-zoster virus, is contagious to people without immunity either through previous infection or vaccination. It is transmitted through direct contact or respiratory droplets. Shingles, which is a reactivation of the same virus, is not contracted from another person's shingles but can itself lead to chickenpox in susceptible individuals.
Step-by-step explanation:
The correct answer to the question is (a) Chickenpox. Chickenpox is highly contagious to people who have either never had the disease or those who have not been vaccinated against it. The causative agent of chickenpox is the varicella-zoster virus, which can spread by direct contact or through the air by respiratory droplets when an infected person coughs or sneezes.
Infected individuals are at risk of developing chickenpox, which is the initial infection with the virus before it enters a dormant stage in the body. Children often have mild symptoms, but in adults, the disease can be more severe and can lead to complications such as pneumonia or birth defects if a pregnant woman contracts the virus.
Shingles, on the other hand, is a condition that can occur in individuals who have previously had chickenpox. After recovering from chickenpox, the virus remains dormant in the body and can reactivate later in life to cause shingles.
Shingles is not caused by initial contact with someone with the disease, but by reactivation of the varicella-zoster virus already present in a person's body.