It's tricky to definitively say which organism is the largest that can cause infection, as "size" can be measured in different ways. However, here are some contenders:

* Parasitic worms: Some parasitic worms, like tapeworms, can grow incredibly long (up to 30 feet!). While they don't technically "enter" the body in the same way as bacteria or viruses, they are ingested and can live within the host's intestines, causing significant harm.

* Fungal infections: Certain fungal infections, such as candidiasis, can spread widely within the body and form large colonies, visible to the naked eye.

* Protozoa: Some protozoa, like Toxoplasma gondii, are single-celled organisms but can be relatively large compared to bacteria and viruses. They can form cysts in various tissues.

Ultimately, the "largest" organism causing infection depends on what you consider "large." It's more important to focus on the harm an organism can cause rather than its size. Even microscopic bacteria and viruses can be deadly if they cause severe infections.

Let me know if you have any other questions!