There are billions of microorganisms (=microbes) around us, well actually the estimation has so many zeros that it is difficult to imagine. The total number of bacteria and archaea are estimated to ~10^30, the atmosphere contains ~10^22 microbial cells, and terrestrial and marine environments each comprise ~10^29 microorganisms. A human body harbors at least 10^14 microbial cells and 10^15 viruses. These microorganisms can have very various functions and impacts on the environment, on animals, on plants, and on us humans. A microorganism (= biological agent) that causes disease or illness to its host (plant, animals, humans) is called a pathogen.
Pathogens differ in their virulence (how fast they multiply in a host and causing harm) and their pathogenicity (ability to produce pathologic changes or disease), both linked to spread and mortality rate. In the wild, pathogens can cause great harm to populations, ravaging numbers of e.g. amphibians (due to chytridiomycosis and the pathogens Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans), in bats (due to white-nose syndrome caused by Geomyces destructans) or in ash trees (due to ash-dieback caused by Hymenoscyphus fraxineus).
Pathogens could infect individuals of one species (high host specificity) or of many species (low host specificity = generalist pathogen). Interestingly, in a population not all individuals are equal when this population is invaded by a pathogen. Some individuals will only develop few symptoms, when others will die. Some individuals may not develop the disease at all, because they are either resistant or tolerant to the pathogen, thanks to their genetic background, or because they already were infected in the past with a similar pathogen and still have antibodies (a kind of vaccination). Mounting an immune response against pathogens costs energy to the body, thus individuals who are weaker, in less good condition, or already with a lower health status have an increased likelihood to get infected by a pathogen, develop a disease and die. Hence, after a disease has passed through a population, the losses of the weak or less adapted individuals will render the overall population stronger (= better adapted). Pathogens are therefore considered an important selective force in nature. They are actually an integral part of nature and biodiversity and drive evolution due to their selective infection patterns.
Social advances such as food safety, hygiene, and water treatment have reduced the threat to the human population from many pathogens. Medical advances further safeguard against infection by pathogens, through the use of vaccination, antibiotics and fungicides. Some diseases, for example, have been eradicated from the face of the earth. While these advances have helped the human population to proliferate, humans as well as wildlife is not rid of pathogens. The latter evolve, meaning they change to overcome barriers, such as our immune system (and that of animals). It is an arms-race and most importantly in the wild as well as in the human population, pathogen spread is much linked to density. The higher the density, the higher is the probability of propagation. The more intensive and more frequent the contact with wildlife, the higher is the probability that a pathogen jumps from the wild into the human population. So, pathogens will continue to threaten life on Earth and that is actually their job.