x is a contiributing factor toin the spread theing of disease. x is a contributing factor in the spreading of disease.

xX is a contiributing factor in the the spread of the disease. X is a contributing factor in the spread of the disease.

This is a good question! I agree with the previous commenter. This sounds the most natural. The word “contributing” has positive connotations, but even America’s Centers for Disease Control and Prevention use that word, so I don’t think that’s bad. Your sentence does sound formal, like something in a newscast or a news article. In informal conversation, I would say something like, “The disease is spreading, and it doesn’t help that X.”

wWhich one is correct? Which one is correct?

hHow todo I use the word factor? How do I use the word factor?

x is a contiributing factor in the the spread of the disease. x is a contributing factor in the spread of the disease.

With “factor” I would go with “in”, but I would really prefer to omit “factor”: X contributes to the spread of the disease.

x is a contiributing factor to the spread of the disease. x is a contributing factor to the spread of the disease.

This is the closest to being acceptable. You could say something like "Close-quarters sleeping arrangements within densely-packed apartment buildings are a contributing factor to the spread of the disease." I would understand what you meant if you said this. However, from an epidemiology (the study of diseases) point of view you would most likely want to indicate what increases or decreases the rate of spreading the disease. Spread of a disease between two organisms is termed "transmission". So a more correct sentence would be "Close-quarters sleeping arrangements within densely-packed apartment buildings are a contributing factor to an increased rate of disease transmission."