Like AIDS, flu is caused by a virus, a tiny packet of genes in a protein coat so simple that most biologists don’t     1     viruses truly “alive” until they invade（入侵）a host cell. Then they hijack the cell and use its energy to     2     out 100,000 to 1 million copies of themselves in just 10 hours. Eventually they become so great in number that they     3     the cell from within, flooding out to further infect the body. They are spread from respiratory（呼吸的）passages, through the cough and sneeze droplets on doorknobs, desktops, coat sleeves and food tables.

The cause of flu was unknown for many years—the virus wasn’t isolated until 1933. But now we know what makes flu such an annual irritant, and so     4     deadly, is that like HIV that causes AIDS, it is moving     5    . Made of RNA instead of the more stable DNA, it changes like a movie monster, making vaccinations（疫苗接种）useless in a year or so.

One thing we know about the flu is that we can’t predict it. In part that’s because occasionally the flu will change completely in what is called a “shift” and     6     humans by such a surprise that a pandemic（流行疾病）breaks out. An estimated three to six shifts occurred in the 18th century, four in the 19th, and three in the 20th, the 1918 pandemic, the 1957 Asian flu and the 1968 Hong Kong flue. Because it can take six to nine months to make enough of an effective vaccine, a deadly shift that     7     around the world is what gives health authorities nightmares.

But there is a global network to try to stop such a pandemic in its tracks. More than 80 countries and 121 U.S. cities belong to a reporting system to     8     flu.

And now, doctors know what causes flu. They have decades of experience with vaccines to fight it, and they even have anti-viral drugs, though those are expensive and of limited     9    .

Autism Spectrum Disorder

Autism Spectrum Disorder, or ASD, affects how people communicate with and relate to others. Most people with autism don’t understand some of the basic social     1     that others take for granted. They might have trouble making eye contact, holding a conversation, or recognizing gestures. And over one-third of people with ASD are nonverbal, meaning they don’t use speech. Along with communication     2    , people with autism often like to follow certain patterns or     3     behaviors. Many are sensitive to bright light or loud noises, and others have physical problems, like trouble walking or picking up small objects. Some have     4     disabilities, but about half have average or above average IQs. It’s also common for people with autism to have a great long-term memory for certain details, and many excel in math, science, music, or art.

With such a wide variety of symptoms, no two people with ASD are alike. The behaviors vary so much that they used to be     5     as different disorders. One was Asperger Syndrome, where people obsess over particular topics,     6    nonverbal social cues, and may not understand appropriate social behaviors.

Even though there’s no cure for ASD, therapy and medication can help people adjust. Scientists are also doing clinical     7     to find other solutions. They’ve learned that 1 out of every 68 children in the US has the disorder, but they still aren’t sure what causes it. No matter why it happens, ASD is being     8     at a higher rate every year. This doesn’t mean it’s becoming more common. It just means more people are aware of the condition and getting professional help early, and awareness is     9    . The more we learn about autism, the more we can understand and relate to those who have it.

It’s important to note that people with ASD deserve the same respect, fairness and chances that people without ASD receive. This will help people with ASD     10     and grow in our communities.