Considering the dangers associated with spreading a disease, why are people who are affected by communicable diseases told not to travel on airplanes?

a. the high altitude contributes to the spread of the disease.

b. the pressurized cabin can result in death for people infected.

c. the dry air in the cabin makes the symptoms more severe.

d. the tight cabin space makes it easier for the disease to spread.

An airplane is a closed space, where air is recycled- people breathe in what other people breathe out, because the ventilation happens very slowly. So, people are using the same air for hours, which means that the germs in the air have a higher chance to reach other people than in the normal atmosphere - where the germs spread by a person are mixed in with the huge amount of air in the whole atmosphere. 

So the answer is: D.The tight cabin space makes it easier for the disease to spread.

because the air is recycled. 

In addition, sitting next to other people means that it's more likely they will cough at you, which again leads to the spread of the germs.