[Music] thank you there are many important pathogens to learn about that have had large impacts on societies over the course of human history and for the IB examination you need to know a few the first one is a virus called HIV which stands for human immunodeficiency virus HIV is a nasty virus that can affect the human immune system by targeting T cells HIV infiltrates and kills T cells while producing more copies of itself which means that our third line of defense cannot work properly with no T cells around the B cells cannot be activated in the same way for the Adaptive immune system to work this leaves our body defenseless against other pathogens that would normally not be a problem for our body to fight off this graph shows what happens to the number of T cells once a person gets infected by HIV at first there is a dramatic decline in T cells that brings about symptoms of acute HIV syndrome then the virus goes into a dormant State during the clinical latency phase only to become active again later with which can ultimately lead to death of the human host HIV can be transmitted via unprotected sex pregnancy and breastfeeding sharing of needles exposure to infection if you work in the medical field or by blood or organ transfusions as we mentioned on the previous slide if someone has HIV it can lead to the breakdown of their adaptive immune system if this happens and the HIV virus has destroyed most of someone's T cells to the point that they can no longer defend themselves properly from other pathogens we call this condition AIDS AIDS stands for acquired immunodeficiency syndrome which is basically the condition your body is in at the third stage of having the HIV virus if someone dies from AIDS it is generally not the HIV virus that kills them but a different pathogen that enters their body but because their immune system does not work properly there is no effective way to destroy other pathogens like the common cold the flu or other fungi that can cause pneumonia that can be extreme extremely lethal for somebody who has AIDS these other pathogens are called opportunistic infections because they take advantage of the weak immune system of the host who under normal circumstances could naturally fight against them antibiotics are chemicals that kill or inhibit the growth of bacteria by targeting prokaryotic metabolism a common example of an antibiotic is penicillin penicillin was identified by Alexander Fleming in 1928 on accident this chemical came from a mold called penicillium and when placed next to other bacteria he noticed that it inhibited their growth years later two other scientists named Ernest chain and Howard Flory infected mice with pathogenic bacteria to see if penicillin would destroy the pathogen and cure the host with an experiment of only using eight mice they were successful and started to immediately treat humans that had bacterial infections which is a pretty quick turnaround time that would not fly today antibiotics only work at stopping bacterial infections and not viral infections viruses do not have a cell metabolism of their own along with other distinct prokaryotic features which means that these chemical compounds have no effect in stopping them this is where antiviral medication comes into play more on that later the use of antibiotics while it may be good also comes with a price bacteria are evolving because of increased antibiotic use and now carry genes that make them resistant to antibiotics this is a double-sided coin because using antibiotics can potentially cure us of infections but also gives bacteria an opportunity to evolve and build resistance we as conscientious Citizens need to use antibiotics properly which means following the directions and taking all of the medication without stopping early and only use them when necessary to preserve the high effectiveness of the drugs we create while we do have the ability to synthesize new antibiotics the process is slow and tedious and does not always yield successful results recently scientists have been using chemical libraries to discover new combinations of antibiotics which means they are searching through all of the chemicals we know of and their properties to see if that can potentially fight off bacteria that are resistant to the other commonly used antibiotics today there are a few known bacterial species that are resistant to all known antibiotics which is a scary thought leading into our medically Advanced future many of the pathogens that we know of are specifically designed to infect one species so we should not generally see a pathogen that infects beetles also infect Birds while this is usually the case there are exceptions to the rule with some pathogens having the ability to transfer hosts from one species to another pathogens that can move their way from other animals into humans are classified as zuanasis there are four specific examples of zoonotic infections that you need to know of for the IB exam the four are tuberculosis rabies Japanese Encephalitis and more recently covid-19 let's talk about each one individually tuberculosis is the only disease on the list that is caused by a bacterial infection from the bacteria species mycobacterium bovis if this bacteria gets into your body it can cause damage to your lung tissue among other tissues within the body humans can get infected by m bovis through exposure to cattle which includes Airborne transmission eating infected meat or consuming unpasteurized milk products that are infected by the bacteria the next three zoonotic diseases the rabies Japanese Encephalitis and covid-19 are all caused by different viruses that can make their way into humans from other animals rabies is caused by the rabies virus abbreviated rabv and is commonly passed through direct contact with animals like a dog bite this virus causes inflammation of the brain and spinal cord which can lead to severe illness and death the Japanese Encephalitis virus abbreviated je can be transferred to humans by a mosquito bite this is specifically transmitted via one species of mosquito culex tritinio rinkus this mosquito usually receives the virus from another animal like a bird or a pig and then passes it to humans after it has picked it up South southeast Asia is where most cases of this disease occur which generally cause mild symptoms but can progress to be more serious with some people leading to Comas and even death lastly covid-19 is a disease caused by a type of coronavirus called SARS Cove II at the date of this recording the transmission animal that humans received it from is unknown though it is probable that it is zoonotic because all other known types of coronaviruses can be zoonotic in addition we have found other animal species that have tested positive for covid-19 including dogs cats deer and more the symptoms of the SARS Cove 2 virus vary greatly with some people being infected but showing no symptoms asymptomatic and others suffering from severe and sometimes fatal respiratory damage advances in technology and a deeper understanding of how immunity and pathogens work has led to advances in how humans can protect themselves against the ever-evolving threats that seek to infect us one major break through with fighting disease has been the creation and use of vaccines earlier we discussed how adaptive immunity works and vaccines play off this process normally during the first exposure a pathogen would enter the body and the antigens are recognized as foreign and the process of T and B cells work to produce antibodies to destroy it then the memory B cells hang around just in case that same pathogen enters again with vaccines we can activate this process of adaptive immunity without ever having an active pathogen enter the system we do this by using their antigens there are different types of vaccines but the main idea is that the antigen found on the pathogen enters the system without the pathogen being able to infect the host from there the body will go through the normal process of creating antibodies and memory B cells to fight the pathogen as if it were undergoing an actual attack which it isn't this would mean if you have taken the vaccine and get infected at a later date your body would be treating it as a a second exposure even though it was technically the first exposure with an active pathogen this would make the process of fighting the pathogen much faster and efficient because the body already has the memory B cells and tools to beat it vaccines can work in a few different ways first an inactive version of a pathogen can be put in the vaccine which contains the antigens second just the antigens can be added into the vaccine that the body will recognize as foreign or third specific sections of DNA or RNA can be put in the vaccine our cells can use either one to create the antigens with the Machinery they already have once made they would be treated as a foreign antigen and the immune process would begin these vaccine methods are especially important for protection against viruses like the three we mentioned on the previous slide because we do not have other medicines like antibiotics to fight them off like we do with bacterial infections while vaccines are helpful in building resistance to specific diseases there are other statistics that show Once a population reaches a certain threshold of immunity the transmission of the disease is greatly reduced which protects everyone including those who do not have immunity we call this phenomenon herd immunity so if we have 10 people in a room and nine of them can easily fight off the infection from either natural memory B cells or B cells built up from a vaccine we would call them immune that one other person does not have any immunity and if they were to get the disease could actually become very sick and potentially die from it but based on 90 percent of the group being immune it is unlikely that they will get it because if any of the other nine people contract the disease they will be able to fight it off quickly through the second exposure to the point where they will not be capable of passing it to another person data on disease transmission and herd immunity can often be misinterpreted while in the middle of a pandemic like we saw with covid-19 this is because as scientists publish their research so that other scientists can evaluate it the media often report on the research while evaluation is still happening and consumers need to be aware of this vaccines are tested rigorously and the risks of side effects are minimal but not zero the distinction between practical truths and certainty is poorly understood being able to evaluate data is an important skill for a scientist especially when it relates to information as important as the spread of disease let's evaluate some covid-19 data using two calculations percent difference and percent change when measuring the percent difference of two numbers we subtract the two numbers and divide by the average of the two numbers and express the answer as a percentage this is most helpful when comparing two sets of data that are representative of the same thing at the same time percent change on the other hand is calculated by subtracting a new value from an old value and dividing by the old value also expressed as a a percentage this is useful when comparing two values of the same thing that were collected at different times let's work through a hypothetical example let's say that for the population of Chicago to have successful herd immunity for disease a there must be at least 2.2 million people who are immune and the herd immunity for disease B requires 1.8 million people to have herd immunity what is the percent difference between 2.2 million and 1.8 million people to answer that we can take 2.2 million minus 1.8 million which gets us 400 000 and divide that by the average of the two numbers which we add them together and divide by two to get 2 million four hundred thousand divided by 2 million gets us an answer of 0.2 we multiply that by 100 to get our final answer of 20 so we can say there is a 20 difference between the population requirements for herd immunity between diseases A and B for the population of Chicago now let's say that these numbers were accurate for calculations done in 2025 but in 2030 a new variant of the same pathogen that caused disease a requires 2.5 million people to be immune for there to be successful herd immunity what is the percent change in immunity required for the population to achieve herd immunity from disease a from 2025 to 2030. to answer that we can take 2.5 million and subtract 2.2 million to calculate the difference and we get 300 000 then divide that by our old value of 2.2 million to get 0.14 which is rounded from 0.136 we multiply that by 100 to get our final answer of 14 so this tells us that there is an increase of 14 percent of people who need to be immune this time around to achieve herd immunity in 2030 compared to what it was in 2025 knowing the difference between these two equations and when to use them is important as they can and most likely will show up on the IB exam [Music] thank you [Music]