It is a mystery that has baffled doctors for decades.

But experts believe they have now finally solved the tricky¹ question of why winter is notoriously known as the flu season.

According to new research, the influenza² virus coats itself in a protective fatty shield that is tough enough to withstand cold temperatures.

The butter-like material only melts when it hits the respiratory tract³, leaving the virus free to infect cells - a process scientists say is like an M&M melting in the mouth.

However, in warmer outdoor temperatures, the protective coating melts before it reaches a person or an animal, killing⁴ the germs before they can infect someone.

Joshua Zimmerberg, from the National Institute of Child Health and Human Development (NICHD), in the US, which led the study, said: 'Like an M&M in your mouth, the protective covering melts when it enters the respiratory tract.

'It's only in this liquid phase that the virus is capable of entering a cell to infect it.'

In the past scientists have worked on theories that flu is more common in winter because people spend more time inside or the radiation from the sun in summer kills off germs.

But no research has successfully explained the spread of the disease.

Researchers have already claimed that the report, which is published in the journal Nature Chemical Biology, could lead to new ways to prevent and treat flu.

Duane Alexander, the director of NICHD, said: 'The study results open new avenues of research for thwarting⁵ winter flu outbreaks.

'Now that we understand how the flu virus protects itself so that it can spread from person to person, we can work on ways to interfere⁶ with that protective mechanism⁷.'

Influenza and other respiratory viruses are spread in small droplets⁸ broadcast by coughing, sneezing and talking and which can also settle onto surfaces, to be picked up on fingertips.

为何冬季为多发流感？

这可是困扰了医生们几十年的一个谜。

不过，专家们现在终于解开了这个难题。

一项最新研究发现，流感病毒表面包有一层脂肪保护外壳，这种外壳十分坚硬，能抵御严寒。

这种油状物质在进入呼吸道后才会融化，病毒被释放后即会感染细胞。科学家称这一过程如同M&M巧克力豆在嘴里融化。

然而，在温度较高的环境下，这层保护壳在进入人体或动物体内之前就会融化，因此病毒在感染细胞之前即被杀死。

研究负责人、美国儿童健康与人类发展研究所的约瑟华•兹姆博格说：“这层保护壳在进入呼吸道时即会融化，就像放入口中的M&M巧克力豆一样。”

“而就在这一液态阶段，病毒才得以入侵并感染细胞。”

科学家们过去一直在研究冬季易发流感的原因，一些理论称这是因为冬季人们多在室内活动，或是因为夏季的阳光辐射会杀死细菌。

但没有一项研究能真正解释流感传播的原因。

研究人员们称，这项研究成果能为防治流感提供新办法。研究报告将在《自然化学生物学》期刊上发表。

美国儿童健康与人类发展研究所主任杜安尼•亚历山大说：“研究结果为控制冬季流感爆发开辟了新途径。”

“现在我们明白了流感病毒是如何保护自己并在人际间传播的，因此我们可以研究如何‘干扰’这种保护机制。”

流感及其他呼吸道病毒主要通过咳嗽、打喷嚏或说话时散播的飞沫传播，它也会附着在物体表面，通过被污染的手指接触传染。