The types or groups of human pathogens include bacteria, viruses, fungi, protozoa, worms and prions. All of these pathogens are responsible for causing disease in human hosts, but some are more common than others.
Within the human body, the presence of any number of pathogens is quite common and does not necessarily lead to illness. However, it is once these microbes begin to multiply and damage the cells of the human host that true illness or disease occurs. It is at this point that the host must take action against an individual pathogen according to its type.
While an individual pathogen may be a member any of the aforementioned groups of organisms capable of infecting the human body, viruses and bacteria are by far the most common.
Viruses affect a human host in two ways: killing cells and disrupting their normal function. Viruses are not actual cells or organisms, but use the host to replicate and survive. Therefore, the human body itself is the best line of defense against a viral infection. It reacts most often with a fever because heat inactivates many viral processes. It also releases a chemical, interferon, which prevents viruses from reproducing, along with antibodies designed to attack the intruding virus.
When a bacterial infection occurs, the offending strain kills cells and disrupts function like a virus. However, certain bacteria can also kill tissue completely, destroy cellular metabolism, and cause a toxic immunoresponse. Because bacteria represent a unique kingdom of living organisms separate and distinct from their host, antibacterial drugs are used to prevent the replication of bacterial DNA without affecting the same processes within the host itself.