The biggest thing an average person might introduce is something like germ theory, which could be pretty massive - but in order to get people to take your advice of washing hands and boiling water seriously, you would have to rise through the ranks of the medical establishment as it exists, which depending on time and place probably means climbing the ladder of whatever is the dominant religious institution. Only after decades of accruing social capital would you have any hope of being seen as anything but one of about a million cranks with your wild and untestable theory of “tiny animals that can’t be seen”.