While the forms of infection that we have listed above are the main types, there are others that can have an effect on the body. A single celled organism with a nucleus can cause a protozoan infection. Protozoa commonly show features similar to those of animals, such as mobility, and they can survive outside the human body. They most commonly transfer to other humans through feces. Amebic dysentery is an example of a protozoan infection. Helminths are larger, multicellular organisms that tend to be visible to the naked eye when fully grown. This type of parasite includes flatworms and roundworms. These can also cause infection. Finally, ectoparasites — including mites, ticks, lice, and fleas — can cause infection by attaching or burrowing into the skin. Ectoparasites can also include blood-sucking arthropods, such as mosquitoes, which transmit infection by consuming human blood. The cause of an infection is whichever type of organism has entered the body. A specific virus, for example, will be the cause of a viral infection. The effects of an infection, such as swelling or a runny nose, occur due to the immune system’s attempt to get rid of the invading organism. A wound fills with pus, for example, when white blood cells rush to the site of an injury to combat foreign bacteria.