Parasites ubiquitous. The majority of living organisms are parasitic in one form or another and they're the predominant mode of life on the planet. Since almost all living organisms are parasites, even parasites have parasites. This was nicely caught, in this little ditty. Great fleas have little fleas upon their back to bite 'em, and little fleas have lesser fleas, and so ad infinitum. >> There are two ways to think about the types of parasites we encounter. The first is a taxonomic classification that divides all the pathogens and parasites into taxonomic groups, depending on the relationship with you other organisms. These groups include viruses, bacteria, helminths, fleas, ticks, etcetera. The categories are based on how related they are by common descent and not by how they appear. So, bats and birds fly, but they are in different groups because one is a mammal and the other's a bird. >> The second is to make a functional division that reflects fundamental biological features. And in this case, we simply divide them into two broad groups known as the micro parasites, and the macro parasites respectively. >> As the name implies, micro is small and macro is larger. But more that just being small versus large, the different categories have very different ways of life. These two groups have very different biological characteristics. Which means their epidemiology is very different. The microparasites replicate inside the host, whereas the macroparasites have a special transmission state, outside of the hosts. And with macro parasites, every parasite has entered the host body in you, and there is no replication within the host. As such, the impact of macro-parasites is really a function of the number of parasites that are within the host. With just a few, the host may be able to tolerate the infection. But the more you have, the sicker you feel. And if you have many parasites the likelihood is you're going to die. One consequence of this, is that hosts often have a moderate level of infection. So the parasites reduce condition and tend to cause more morbidity rather than mortality. Now that word morbidity, is a wonderful old English word, that means the unhealthy symptoms of lethargy associated with disease. And it is this, that is characteristic of macroparasite infections. This morbidity means the impact, the parasites have on the host, tend to be sublethal. So the parasites reduce the competitive ability of the host. Reduce their ability to rear their offspring. And even though the infection is not in their young, it can result in the young not being healthy, or even dying. The macroparasites tend to incite a complex immune response, and since the moderate to lower burdens are tolerated, the infections can be persistent and last for most of your life. In contrast, microparasites give you a very short-term infection that leads to a crisis from which you either die or recover from. Indeed there is one interesting study on a parasitic tapeworm of a rat, where the tapeworm has survived longer than its host. And the researchers have taken the tapeworm and moved it from one rat to another rat to another rat. And successfully transplanted at least five times. And the tapeworm has continued to live while of course the hosts have died. >> A useful example of a microparasite infection would be the cold virus that reproduces inside your body by turning your cells into viral factories releasing more virus. Since we mentioned viruses, let's begin with these. These are very small, and many hundreds of times smaller than the human cells they infect. They only develop within whole cells. Many human diseases such as the common cold, HIV, measles, and herpes are due to viruses. But there are also parasites of lots of other types of life such as bacteria, fungi, plants and non-human animals. Even smaller than virus are prions, which are proteins that are folded in an abnormal way which allows them to become infectious. Many prions are not parasitic, but some are. And the name derives from these, proteinaceous infectious. They can cause devastating diseases such as scrapie in sheep, mad cow disease, and the debilitating diseases of humans called CreutzfeldtâJakob disease, and Kuru. The last one, Kuru, which translates as a laughing disease, in a Papua New Guinea language, is spread as people eat the brains of others. Best avoided. Prions are definitely are not alive, but interestingly, they can ev, evolve. And our genome carries a signature of co-evolution as we evolve defenses against them. Next in our list are bacteria. Bacteria are typically much larger than virus. Although there is an overlap, with the largest virus being larger than the smallest bacteria. Many bacteria are highly beneficial to humans, and others live on us in a commensal fashion, neither harming nor helping. Fungi are also microorganisms because they can occur in a single cell stage. Although when you see or eat a mushroom you would be forgiven for not thinking it was a micro anything. We humans daily inhale millions of fungal spores and our immune system is constantly battling them. For AIDS patients whose immune system breaks down, they're often killed by fungal disease that would be benign in a healthy adult.