Did you know that your body is an ecosystem of more than 1000 different bacterial species?!? It may be hard to believe, but scientists estimate that there are about 10 times more bacterial cells than human cells in our bodies. Many of the microbes that live within us are important to our health and nutrition, and allow us to do things that we may not be able to otherwise.

Did You Know?
Infection by Yersinia pestis, the supposed cultprit behind the infamous "Black Death", was responsible for the death of nearly one-third of Europeans in the 14th century. Y. pestis was able to infect and kill so many people because it stops the body from being to digest Y. pestis cells, allowing the population to increase exponentially to a level that the body cannot sustain.

While we benefit from the existence of these helpful microbes, we are constantly in battle with an army of nasty disease-causing microbes in the environment. For example, Salmonella is probably all over your kitchen, just waiting to cause your little brother to have nasty case of diarrhea...

Strains of Streptococcus, which cause strep throat, like to hang out in the throats and nasal passages of you and all your friends, quietly preparing for warfare...

And those bathroom doorknobs? Well they're home to a whole whack of other types of bugs

Did You Know?
A vaccine contains information about a pathogen that your body uses to manufacture antibodies. Vaccines are administered by your doctor so that if you ever encounter that pathogen for real, your body has a head start and can fight it off quickly.

So buck-up...that shot really could save your life. But, for the most part, our healthy days out-number our sick ones. This is because we've got an immune system that is swift at clearing most infections.

Immunity responses are first activated by the recognition of certain molecules that are common to microbes but that are not found in our own cells. Our cells have evolved the ability to recognize these pathogen-related molecules via receptors — proteins on the surface of a cell. This recognition leads to a series of events ending in the secretion of inflammatory molecules that alert the rest of the body to the threat of infection.

More specifically, these molecules draw the attention of specialized immunity cells called macrophages. These cells absorb invading pathogens by encasing them in sacs made of their own cellular membrane. These sacs later fuse with digestive vesicles called lysosomes that process the pathogen and release information about its identity to the rest of the body.

Eventually, this information is used to develop remarkable surveillance proteins called antibodies. Antibodies identify other bacteria in the invading population, and attract macrophages to the scene of the crime to engulf and digest them. This response is usually so quick that we may not even notice that we've been infected.

But, every now and again, our surveillance system doesn't spot the infection right away. In these cases, the pathogen has a little time to set up camp, get settled in, and start reproducing. This means that once we finally do notice the invaders, we have more of them to fight off than if we'd noticed sooner. It also means that we need to put a lot of energy into the battle between our cells and the invading cells if we are to win.

Did You Know?
Because our immune systems are so good at fighting bacteria, they, and other microbes such as viruses, have had to be resourceful if they want to win the war against us. These small creatures evolve much more quickly than we do, and they use this to their advantage by morphing into something that our bodies aren't able to recognize. So, it is likely that you have fought off the same pathogen—only, slightly different—more than once.

Our bodies often use fever as an attack against trespassing pathogens. Some pathogens find an increase by even a few degrees in our body temperature to be unbearable. So, good news to those of you who have a fever: you will win this battle soon.

Learn More!

Coombes BK, Valdez Y, and Finlay BB. 2004. Evasive maneuvers by secreted bacterial proteins to avoid innate immune responses. Current Biology 14: R856-R867.

Hooper LV, Gordon JI. 2001. Commensal host-bacterial relationships in the gut. Science 292: 1115-8.

McGill University Student Health Service. 2007. Fever. Sears CL. 2005. A dynamic partnership: Celebrating our gut flora. Anaerobe 11(5): 247-251.

Jacqueline Monaghan was born and raised in Toronto, Ontario. She obtained her Hons. B.Sc. from the University of Toronto in 2005, and is currently working towards her Ph.D. at the University of British Columbia. Her thesis explores the molecular genetics of plant innate immunity.

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