阅读下面短文，并根据短文后的要求答题（请注意问题后的字数要求）。
[1]Global difference in intelligence is a sensitive topic, long filled with a large number of different opinions. But recent data has indeed shown cognitive (认知的) ability to be higher in some countries than in others. What's more, IQ scores have risen as nations develop—a phenomenon known as the "Flynn effect". Many causes have been put forward for both the intelligence difference and the Flynn effect, including education, income, and even non-agricultural labor. Now, a new study from researchers at the University of New Mexico offers another interesting theory: intelligence may be linked to infectious-disease rates.
　　[2]The brain, say author Christopher Eppig and his colleagues, is the "most costly organ in the human body". Brainpower consumes almost up to 90 percent of a newborn's energy. It's clear that if something affects energy intake while the brain is growing, the impact could be long and serious. And for vast parts of the globe, the biggest threat to a child's body—and therefore brain—is parasitic (由寄生虫引起的) infection. These illnesses threaten brain development________________. They can directly attack live tissue, which the body must then try every means to replace. They can invade the digestive pipe and block nutritional intake. They can rob the body's cells for their own reproduction. And then there's the energy channeled (输送) to the immune system to fight the infection.
　　[3]Using data on national "disease burdens" (life years lost due to infectious diseases) and average intelligence scores, the authors found they are closely associated. The countries with the lowest average IQ scores have the highest disease burdens without exception. On the contrary, nations with low disease burdens top the IQ list.
　　[4]If the study holds water, it could be revolutionary for our understanding of the still-confusing variation in national intelligence scores.
What is the main idea of the text？(no more than 10 words)________________________
Complete the following statement with proper words.(no more than 4 words)
　　Those countries that have the ________________ are always at the bottom of the  IQ list.
Fill in the blank in Paragraph 2 with proper words.(no more than 5 words)
___________                                                                
What can cause intelligence difference？(no more than 8 words)
　　________________________________________________________________________
What does the word "they" (Line 2, Paragraph 3) probably refer to？(no more than 8 words)
________________________________________________________________________