 What's common between the COVID-19 pandemic that caused global havoc? And chickenpox that many of us would have gotten when we were young. Well, both are diseases caused by viruses. What are viruses? Viruses, as we know, are small, acyllular pathogens. The structure is quite simple. They have a protein coat inside which their genome is present and their genome may be RNA or DNA and it can either be single-stranded or double-stranded. Viruses are very small. They're so small that some, like this T4 bacteriophage, even infect bacteria and bacteria, as we know, are unicellular. What if I were to tell you that there are particles smaller than viruses that can still cause diseases? They are the sub-viral particles. So far, there have been two sub-viral particles identified. They are viroids and prions. Let's first take a look at viroids. Viruses at least have a protein coat surrounding their genome, right? Viroids don't even have that. They are made up of just single-stranded RNA and the single-stranded RNA exists in a circular form. But even this small single-stranded RNA is capable of causing infections. So far, all the viroids that have been identified are known to cause diseases in plants, especially in flowering plants. No known viroids cause human diseases. T.O. Deener was the first to identify viroids. He was studying what was causing the potato spindle disease. He realized that the pathogens causing this disease were much smaller than viruses and even lack the protein coat that viruses had. He coined the term viroids because of the lack of the protein coat. Now, we know that viroids are much smaller than viruses, right? Their entire single-stranded RNA is made up of just 250 to 400 nucleotides. For comparison, the SARS-CoV-2 virus which caused the COVID pandemic has about 30,000 nucleotides. Well, this viroid has just 250 to 400 nucleotides. That comparison in itself should tell you just how small viroids are. Despite their small size, viroids are still of economic importance because they affect a lot of crops. The grape wine Yellow Speckle I viroid affects grapes. The Columnia latent viroid affects tomatoes which causes the tomatoes to grow in an abnormal shape. The next sub-viral particles are prions. Viroids at least have a single-stranded RNA, right? Prions don't even have that. They are just made up of proteins. When proteins are produced, they are folded or structured in a three-dimensional shape. Now, this three-dimensional shape of a protein is very important for its function. If somehow the three-dimensional shape is altered, the protein loses its function. There is a protein called the major prion protein which exists in a variety of tissues including the brain. Prions are nothing but an abnormal shape of this major prion protein. This major prion protein can be written as PRP. It exists in two forms PRPC which is the normal non-infectious form which is found in a variety of tissues and PRPSC which is the infectious or the disease-causing prion form of this protein. Now, this PRPSC generally accumulates in the brain and causes diseases. Keep in mind also that the exact function of this major prion protein is not known yet. We don't know what it exactly does in the body but when it is misfolded or when it becomes into an abnormal shape PRPSC then it causes the prion disease. So, how is this PRPSC formed in the body? When somehow this infectious PRPSC enters the brain it encounters the normal PRPCs. So when this abnormally shaped PRPSC comes in contact with a normally shaped PRPC it causes this normal PRPC to become the abnormal PRPSC. Like this many new PRPSCs are formed. These PRPSCs then begin to accumulate in the brain which leads to neurodegeneration which means a damage of brain tissue. Examples of prion diseases include bovine spongiform encephalopathy or BSC in cattle, scrapey in sheep and the human variant of BSC which is the crufelt Jacob disease. Let's just remember it as CJD because this is quite difficult to say. Prion diseases are progressively worse diseases. Once the symptoms appear they just get worse and worse and as of yet there is no known cure for prion diseases and they are all fatal. All prion diseases lead to death. But there is no need to panic because prion diseases are extremely rare especially the human prion diseases are very very rare. So there is no real need to panic. If you were to find a cure or a treatment for prion diseases where would you start? How would you go about? Here's a hint. We know that this major prion protein has no known function as of yet. Could something be done to this so that PRPSCs are not formed? Ponder on that.