Passage Based Questions

Passage:
The World Health Organisation is briefly called W.H.O. It is a specialised agency of the United Nations and was established in 1948. International health workers can be seen working in all kinds of surroundings in deserts, jungles, mountains, coconut groves, and rice fields. They help the sick to attain health and the healthy to maintain their health. This global health team assists the local health workers in stopping the spread of what are called communicable diseases, like cholera. These diseases can spread from one country to another and so can be a threat to world health. 138. W.H.O. assists different national health authorities not only in controlling diseases but also in preventing them altogether. Total prevention of diseases is possible in a number of ways. Everyone knows how people, particularly children, are vaccinated against one disease or another. Similarly, most people are familiar with the spraying of houses with poisonous substances which kill disease-carrying insects. W.H.O. means:

"International health workers can be seen working in all kinds of surroundings: in deserts, jungles, mountains, coconut groves, and rice fields." Here International means:

They help the sick to attain health and the healthy to maintain their health. Here they stands for:

Read the passage and answer the question:
"Climate change is one of the biggest challenges facing humanity. It affects the environment, economy, and health. Immediate action is required to reduce greenhouse gas emissions."
What is the main idea of the passage?