in this video we're going to take a look at what viruses are and at three of the diseases that they can cause now viruses are quite hard to explain they aren't cells and they're not living but we normally still consider them to be organisms they're about 10,000 times smaller than animal cells and they can't reproduce by themselves however once they get inside another organism set they can use that cell's own Machinery to make loads of copies of themselves and then when the time is right they can burst the cell and all of those new viruses we have to find new cells to take over this process of colonizing and then bursting our cells is obviously going to damage our bodies and that's one of the main reasons that viruses make us feel ill the three examples of viral disease that we're going to look at are measles HIV and tobacco mosaic virus the measles virus is spread by droplets when an infected person coughs or sneezes it generally causes people to develop a red rash across most of their body and a fever which is where you get a really high temperature and generally feel unwell although it can be really serious and sometimes even fatal it's luckily really rare in the UK has nearly everyone is vaccinated against it when they're young h on the other hand can be spread by sexual contact such as unprotected sex or by exchanging bodily fluids such as the blood for example when people share needles HIV stands for human immuno deficiency virus which tells us it affects humans and it causes a deficient or inadequate immune system and it's a bit weird but when you use the term HIV you're referring to the actual virus not to disease when somebody first catches HIV they might get flu like symptoms for a few weeks so fever tiredness and aches but then they'll start to feel better and think that everything's fine however inside their body the viruses are doing more and more damage to their immune system and at some point their immune system becomes so weak that they start to catch unusual infections and sometimes even get cancers at this point we say that the person has developed AIDS which is classed as a disease or syndrome in which the person's immune system can't cope anymore this used to be the end of the story as people would die of random infections or cancer now though we have a group of medications called anti-retroviral drugs which prevent the viruses from replicating within our bodies so as long as people have access to these medications early on in their disease then they can go on to live normal lives the last viral disease we're going to cover is caused by tobacco mosaic virus or TMV this virus affects plants instead but only certain species such as tobacco and tomato plants the reason it's called Mosaic is because it discolors patches on the leaves causing a mosaic pattern to form the problem is that photosynthesis can't take place in these discolored patches and so overall the plant won't be able to produce enough sugars for proper growth if you haven't heard yet you can find all of our videos on our website Cognito dog you'll also find questions flashcards exam style questions and pass papers and we track all of your progress so that you always know what to study next so sign up for free by clicking here or browse our playlist here on YouTube