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Germs are everywhere: in school, at home, at the gym, at the mall, even on your computer! You're constantly exposed to germs. Fortunately for most of us, our immune systems defend us against germs and microorganisms daily to keep us healthy and prevent infection. The following discussion questions and activities will help your students learn all about the immune system.

Germ theory states that specific microscopic organisms are the cause of specific diseases. The theory was developed, proved, and popularized in Europe and North America between about 1850 and 1920. Because its implications were so different from the centuries-old humoral theory, germ theory revolutionized the theory and practice of medicine and the understanding of disease. It was, however, compatible with existing ideas about health, especially those associated with 19th-century hygiene and sanitation.

5:06 video, Germs are found on almost every surface we come in contact with, which makes it incredibly common for our bodies to be exposed to them. But why are some of these germs relatively harmless, while others can be fatal?

Watch the adventures of the inept "Staph Sargent" and his trusty sidekick as they try to infect the world with germs in this animated educational video. But as they plot their evil plan, they are thwarted at every turn by trained PDI Agents (Protect Don't Infect) who thwart their attempts by washing their hands, covering their cough and sneeze and staying home when sick. This wacky video is aimed at kids aged 6-11 but there's plenty of humor that goes right over their head and finds it's target with mom and dad! With worries about the spread of the H1N1 "Swine Flu," this program is perfectly timed for the beginning of school season. The video was produced by the creative team at Omni Productions in Orlando, FL and is part of a very successful campaign by the Orange County Department of Health.

We are under attack - by germs. Drug-resistant bacteria are invading organisms, and hospitals are their favorite breeding ground. Scientists are studying the genetics of bacteria and trying to find

One common virus takes a sneaky route to success. It doesn't kill its leafy hosts. Instead, it makes infected plants smell more attractive to bees. That ensures this germ will have a new generation of the plants to host it in the future.

It's peak cold and flu season, and mucus is making many of our lives miserable. But despite being a little icky, phlegm gets a bad rap. This germ-fighting goo contains cells and chemical compounds that help us power through a cold. You can also think of mucus as a traffic light for your health - what turns up in our used tissues can be a useful clue about the inner workings of our immune systems.

1:30 animation As a human embryo develops, its cells become progressively restricted in the types of specialized cells that they can produce. Inner cell mass (ICM) cells of the blastocyst can make any type of body cell. Gastrula-stage cells can give rise to the cells of a given germ layer. Later, cells become even more restricted. For example, the pancreatic bud of the endoderm layer can only make the cells of the pancreas.

2:18 Human embryonic development depends on stem cells. During the course of development, cells divide, migrate, and specialize. Early in development, a group of cells called the inner cell mass (ICM) forms. These cells are able to produce all the tissues of the body. Later in development, during gastrulation, the three germ layers form, and most cells become more restricted in the types of cells that they can produce.

The most recent human retroviral infections leading to germ line integration took place with a subgroup of human endogenous retroviruses called HERVK(HML-2). The human genome contains ~90 copies of these viral genomes, which might have infected human ancestors as recently as 200,000 years ago. HERVs do not produce infectious virus: not only is the viral genome silenced - no mRNAs are produced - but they are littered with lethal mutations that have accumulated over time. A recent study revealed that HERVK mRNAs are produced during normal human embryogenesis. Viral RNAs were detected beginning at the 8-cell stage, through epiblast cells in preimplantation embryos, until formation of embryonic stem cells (illustrated). At this point the production of HERVK mRNA ceases. Viral capsid protein was detected in blastocysts, and electron microscopy revealed the presence of virus-like particles similar to those found in reconstructed HERVK particles. These results indicate that retroviral proteins and particles are present during human development, up until implantation.

The bacteria in our guts can break down food the body can't digest, produce important nutrients, regulate the immune system, and protect against harmful germs. And while we can't control all the factors that go into maintaining a healthy gut microbiome, we can manipulate the balance of our microbes by paying attention to what we eat. Shilpa Ravella shares the best foods for a healthy gut.

groups of cells in the blastula are synchronized to divide at the same time through mitosis. Some of these cells will eventually give rise to the sand dollar's germ cells. Others will play important roles in various developmental processes, such as cell differentiation, the formation of the digestive system, and the development of the exoskeleton. Despite their different characteristics and roles, all of the sand dollar's cells (except for eggs or sperm) are genetically identical due to mitosis.

NPR's Jason Beaubien explains cholera, a deadly disease spread through water contaminated with sewage, in under two minutes.

Lesson plans from a 5 day summer workshop through the CDC