 Sam and Mike have fallen terribly sick. Sam woke up with a sore throat. Meanwhile, Mike has the worst stomach ache of his life. What happened to them all of a sudden? Why are their bodies reacting this way? And what exactly are their bodies reacting to? Turns out Sam got the flu from a sneezing woman on the bus. She must have inhaled the virus droplets that got released into the air. Meanwhile, Mike is suffering from a stomach infection called typhoid or enteric fever, which he probably contracted from the undercooked chicken sandwich he had for lunch. Your nose and mouth aren't the only ways that a pathogen can enter your body. Some bugs can enter through breaches in your skin. Here, like broken skin and wounds, the tetanus bacteria often chooses that path. Then some enter through insect and animal bites like malaria and rabies, malaria through mosquitoes and rabies through dogs. Even needle pricks aren't safe. The deadly AIDS virus can enter through needle pricks. But that's probably all right. No, that's definitely not all. Pathogens can also enter through your eyes, genitals and even the placenta. But that's only in pregnant women. That means there are multiple portals through which the all of these bugs can enter. Now back to Sam and Mike. The pathogens are now inside them. So what are they doing inside? Trying to kill them, right? Well, not really. You see, they don't want to cause any harm to you. All a pathogen wants to do is thrive and survive. But unfortunately in the process, our insides get damaged. When a pathogen enters your body, it invades your cells and uses the cells machinery to multiply. But is it really that easy to hijack our tissues? No, but these little bugs are frankly quite lucky. They evolved in such a way that they ended up complimenting us. I'll explain. When the flu virus entered Sam, it invaded her throat cells. The flu virus has these locks on its surface. These pointy things which are sticking out of its surface. These are the locks and the keys to these locks are present on the surface of the throat cells, which are these purple thingies which are sticking out out of the throat cells. Now the virus, what it does, it sticks itself or kind of clicks on to these keys which are present on the cell surface of the throat cells. So it's kind of like a lock fitting, a key fitting into a lock. And once it does that, the virus, it drills itself into the cell. Once inside the cell, it uses the cells components to multiply itself. Later on this cell will burst open and all of these viruses, millions of viruses will spill out of the cell. So this virus is using the healthy throat cells as its personal factory and generating virus babies, which are doing the same thing to more healthy throat cells. So these virus babies which are going to come out of the cell will go ahead and infect this healthy throat cell right over here. All of this damage causes pain and discomfort. And if you don't stop these viruses then they will eventually go into your lungs and cause even more damage. Now let's look at Mike. Mike's typhoid bacteria doesn't have it so easy. This bacteria wants to invade the epithelial cells of the stomach as well as the intestine. Now that's a very difficult thing to do because our organs have very different environments which are deadly to most microorganisms. And invading the stomach is the most difficult because of the acid which is present inside the stomach. This acid is so dangerous that if Sam's flu virus accidentally went into her stomach then it wouldn't survive. But then how is Mike's typhoid bacteria somehow getting past this acid? Originally the typhoid bacteria is an acid sensitive bacteria. It shouldn't survive the stomach acid but it's also incredibly lucky. The undercooked meat that Mike had has its fair share of fats and proteins. These fats and proteins help the bacteria to survive the stomach acid. Especially the fat molecules because they form this protective layering or a protective film around the bacteria which helps it to survive the stomach acid. Now not all pathogens can do this however. Some of them have other ways of surviving the acid. For example Helicobacter pylori or H pylori happens to neutralize the acid. And Shigella happens to resist the acid like the acid can't do no harm. Crazy right? Now I feel really bad for these people. But you're probably thinking or wondering by now that why aren't their immune systems doing anything? Are they just sleeping doing absolutely nothing? Well they are doing everything in their power to get rid of these troublemakers. Our immune system consists of a variety of cells like the macrophages and lymphocytes that essentially monitor the conditions in the body and look for signs of trouble. When these cells detect a pathogen they quickly eliminate the bug themselves or produce certain substances to do the deed. Sam's immune system is producing a lot of mucus. The sticky substance which makes her nose very runny. This mucus traps all the virus particles and either throws them out of the body by sneezing or blowing or the mucus goes into Sam's stomach where the stomach acid kills everyone. Now both Sam and Mike have a high fever as they ride out the infection. Why is that? Well a fever is actually an immune response to an infection in your body. The immune cells they release these lipid compounds that shoot up your body temperature. And how does that help? Well viruses and bacteria love your normal body temperature. 37 degree Celsius is their favorite sunbathing climate. So in order to kill them our body dials up the climate to a heat stroke. A higher body temperature makes your body inhabitable for the pathogens. And that's how Sam's immune system almost cures her. Within only a couple of days she's back to her normal self. No more sore throats or runny noses. But what about Mike? Why isn't he not well yet? Well sometimes even after doing everything in their power the immune system falls short. Because some pathogens can hijack your immune cells too. You heard me. Mike's typhoid bacteria can hijack the macrophage cells and multiply inside them. Now mind you these macrophage cells are like tiny assassins of the immune system. They find and kill all kinds of bugs. But somehow they can't do anything about this little nightmare. And that's not all. As the bacteria multiply they end up producing a ton of different toxins which will eventually go and interfere with your bodily processes and functions. That's why Mike's stomach flu keeps getting worse. So how do we cure him then? In such a case Mike's immune system needs a little boost. And we give this boost with the help of advanced remedies like antibiotics and vaccines. In Mike's case antibiotics work the best. Because the right dosage stops the typhoid bacteria from constantly multiplying. And soon they disintegrate and Mike is on his way to recovery. So bacteria and viruses enter us through different routes of entry and cause an infection. Even when they don't mean to. Once inside they invade the cells and multiply thriving and also damaging our bodily processes. That activates our immune system which sniffs out the pathogens like a police force and eliminate them. And sometimes if the immune system starts to struggle we give it a little boost through medicines and other remedies. In a matter of a few days our friends here Sam and Mike are back to being healthy once again. So what's the moral of the story? The moral of the story is prevention is better than cure. The best way to stay safe is to practice good hygiene, proper sanitation and waste disposal and avoid coming in contact with infected people and their personal stuff. Because the last time that happened we ended up staying in our homes for two whole years straight. So let's make sure that never happens again.