Disease causing organisms are referred to as pathogens. Pathogens are microorganisms that cause diseases in plants, animals, and humans. Examples of pathogens include bacteria, viruses, fungi, and parasites. These microorganisms invade the host organism and cause damage to its cells and tissues, leading to illness or death. Pathogens can be spread through various means, such as through the air, contaminated water, contact with infected individuals, or through vectors like mosquitoes, ticks, and fleas.