-When dealing with the human body, you have to talk about the immune system. So first off, it's your body's defense mechanisms. You have barriers. Just flat out stops things from coming in. Your skin is a great physical barrier. Stomach acid, tears are great chemical barriers. Your stomach has a very, very low pH. A pH of maybe 2, or so. So it's very acidic. It will simply destroy a lot of bacteria. Tears, you're going to have different defensive mechanisms in your tears that help to destroy bacteria. Vomiting. You're saying, well hold on...vomiting? A lot of times vomiting is caused by something that is in the stomach, or in the GI tract, the upper GI tract, that is going to be irritating. It could be a toxin. It could be a bacteria. But something your body wants to get out of there. Then you have these things called non-specific defense mechanisms. Non-specific because it does not matter what triggers it. Anything that causes a problem can trigger it. Phagocytosis. That's when you have your defensive cells eating other cells or particles. So, phagocytosis is a cell eating. Inflammation. If you ever had a cut, a bruise, a bump; you know it swells up. Inflammation is your body's defense mechanism to try to help limit further injury or further infection. But then we have some that are very, very specific. You have antibodies and T cells. Well, T cells are a derivative of one of your white blood cells, they're coming from your lymphocytes. The T cells are meant to cause an immune response. They're meant to attack, destroy foreign particles. Well antibodies, they're also coming from the lymphocyte. Now it's not a direct path, there's something else between. But, the idea is, these antibodies coming from the lymphocytes indirectly are going to be able to recognize foreign objects. Could be foreign pathogens, like bacteria or virus. Could be foreign cells that just shouldn't be there. So we have our antibodies and T cells. They are specific because they only recognize what they're programmed for. The example for antibodies, think about...chicken pox. If you ever had chicken pox, or if you didn't. You might have had the chicken pox vaccine. Well, once you've either had the vaccine or had chicken pox; How often do you get it a second time? Not typically, I mean some people will get it a second time. But more often than not, once you get it, that's it. It's because your body has recognized that virus. Your body made antibodies to recognize the virus and then it fights off the virus anytime it sees it. These diseases could be caused by either living or non-living organisms. Now, these living organisms, like bacteria, fungi, parasites, any of those can cause a pathogenic infection. Now bacteria, you've probably heard of. They're one celled prokaryotes, very basic, very simple organisms, things like Ecoli. Fungi, or fungus, unicellular and multicellular, that means they could be one cell or many cells. But they're eukaryotes, a little more complex. Could be a parasite or parasites even, plural. Again, unicellular, one cell or multicellular, many. So any of these. bacteria, fungi, or parasites can cause infection. But there's also nonliving particles. You have viruses and prions. Now a virus is not living, because without a host, a virus cannot survive. The way a virus typically replicates, or duplicates, is by injecting its DNA into a living cell, letting the living cell perform the action. Prions, well they're even smaller than viruses. They're tiny particles that when ingested into the system, either eaten, it can be have a drink, it could be simply inhaled, unfortunately. They potentially could cause pretty severe infections. So we have both living and nonliving that can both cause a pathogenic infection. Here's looking at the left hand side is a eukaryotic cell. If you notice, a eukaryotic cell has a nucleus. It has all these different particles around it: your endoplasmic reticulum, your golgi apparatus, your mitochondria. All the organelles. But in comparison, look how small the bacterium is next to the eukaryotic cell. And the bacteria, all you're seeing: cell wall, DNA, ribosome, plasma membrane. It's pretty small and pretty basic. But even smaller and more basic, are going to be a virus. So for the virus, it's a tiny little dot in comparison to a larger, eukaryotic cell. So we have our viruses, bacteria that are typical animal cells with a eukaryotic cell. All right, so back to these bacteria. We mentioned they're prokaryotic, single-celled. Just one cell. You might found a clump of thousands of them. But each one is its own organism, with a ton of different ways to grow and reproduce. You can find bacteria living in thermal vents in the bottom of the ocean. You kind find them living in the coldest reaches of a mountain peak. You can find them pretty much anywhere. It adapted to live in almost any environment. Now infections, we're looking at pneumonia, tonsillitis, tuberculosis, botulism, toxic shock syndrome, syphilis, Lyme disease, and the list goes on and on. None of these are overly pleasant. Probably, of course this sounds really bad, but one of the more mild versions here is tonsillitis. Any of the rest of them can get pretty severe, pretty quickly. All by these tiny, single-celled bacteria. More often than not, though, all of these can be treated by antibiotics. Now the trick with antibiotics is to one, make sure it is a bacteria. Because a virus, antibiotics doesn't make a difference. But two, it'd be great if they could figure out, okay which antibiotic will be functional against this particular bacteria. So you know that the antibiotic will work, not just kill off all your normal flora or your normal bacteria, leaving the pathogenic one to get larger and proliferate. Well, if you think about antibiotics and viruses don't forget they usually don't work. They're very, very small. Tiny little thing, much smaller than a bacteria. Now I put them under the nonliving category earlier but there technically is debate of whether they're living or non. The reason that it goes non usually, not living, is because it cannot reproduce without a host cell. That means a virus by itself cannot make more viruses. Also, a virus will not have any metabolic activity. So living, nonliving? It's up for debate. A lot of them are heading more toward the nonliving currently. Structure of viruses, DNA or RNA? You will not have both. And this DNA, or the RNA, within a virus is surrounded by this protein coat, it's meant to keep it safe and keep it contained all in the one place. Viruses can cause things like AIDS, HIV- AIDS. Hepatitis, encephalitis, rabies, influenza, colds, warts, chicken pox and there's more than that. What all these share in common, though, is they all can be confected and caused by a virus. How about these prions? Now prions, a lot of time you've never heard of. They pop up every so often, but more often than not, they're not really talked about. Prions are actually just protein. Its normal brain proteins that just don't quite fold properly. So they turn into these little, tiny particles. And once they're misfolded or turned the wrong way, then it become self-propagating. So once there's a couple of them they make more and more and more. Now all this protein that's around is not helping anything. This excess protein debris can actually fill, disable, and potentially destroy any cell it comes in contact with. And here's the scariest part... Prions cannot be cooked away. You can't kill them. Freezing. Doesn't do a thing. They just stay there because they are nonliving. Drying. You can try to dry out the particles as much as you want. But it's not living. No metabolic activity. So really drying, freezing, cooking. None of these have any impact on getting rid of prions. So bovine spongiform encephalitis, better known as "mad cow disease", is caused by a prion. That's why countries are being so cautious when they go through the whole buy process. If you're buying beef from a country that has found to contain prions, a lot of times they'll shut down all sales in that country. Visible fact they're not wanting to introduce the prions into the current market. This goes for Canada, the US, Britain, any of the countries. We also have Creutzfeldt-Jakob disease. Another prionic-based disease. So both these are based on those mutant proteins just building up and causing problems. Now that we've gone through the different types of pathogens, let's figure out how they are transmitted, how they pass along. Well transmitability is how easy that pathogen can go from person-to-person. It could be in a handshake. It could be someone sneezing and the particles are floating around. So there's different types of transmission. There's respiratory, your breathing. When you sneeze, you cough and all of a sudden, the particles are floating around. It could be fecal to oral transmission. You're thinking, hold on, that's just wrong. Well, fecal to oral. How do you know where all the fecal matter is in the world? The fecal matter has to be filtered out at some point. It goes back into the ground and is reused as nitrogen. So it's not saying direct fecal necessarily, but it could be in some roundabout way. It could've been from some animal that decided to have a problem right in your garden. So this whole fecal-oral sounds disgusting. But, it's one of the biggest reasons why you wash your hands after going to the bathroom. Testing doorknobs and bathrooms, you'd be amazed at how much it shows up on those doorknobs what's there. Body fluids. These are things from tears, to blood, to saliva, to any other fluid coming forth from the body. Virulence is the damage it can infect. Some of the high virulence means it'll influct a lot of damage, very quickly. An example is the Ebola virus The Ebola virus can inflict a ton of damage within the first 24 hours, if not potentially kill the person within the 24 hours. Something with a lower virulence could be the common cold. You might have it for a day or two and not even know it, until it builds up enough. Once it builds up, a couple of days later, a lot of times, you're back up and moving. No major problems. So transmitability, mode of transmission and virulence.