Evolutionary medicine
People a long time ago were less clean than humans are now. Being clean has helped people be more healthy, but there are trade-offs to being clean. There are some microorganisms that are important for humans to have so that we can have strong immune systems. Some microorganisms that grow strong immune systems can come from dirt and animals.<ref></span>http://www.microbemagazine.org/index.php Archived 2013-10-17 at the Wayback Machine? option=com_content&view=article&id=4700:a-darwinian-view-of-the-hygiene-or-old-friends- hypothesis&catid=950&Itemid=1301</ref> Without these microorganisms, people can have weak immune systems. This means they could be more likely to get sick.