There is a protozoan parasite called Toxoplasma gondii. This parasite infects many animals through an interesting route.
The first victim is a mouse. An infected mouse loses its fear of cats, leading it to play fearlessly in front of one until it gets caught and eaten (was Jerry a mouse with toxoplasmosis?). It is unclear how it controls a more advanced animal’s brain, but thanks to this effect, Toxoplasma gondii can infect its intermediate host - a cat.
An infected cat starts excreting parasite eggs with its faeces. If a person forgets to wash their hands or eats food contaminated with cat faeces, they can be infected and become the final host for the parasite.
Usually, Toxoplasma gondii cannot overcome the healthy immune system, but it can infect those with a weaker immune system such as the elderly or pregnant women. Furthermore, it is part of the TORCH complex (toxoplasmosis, rubella, cytomegalovirus, herpes) - a group of infections that commonly cross over from the mother to the fetus in utero. Symptoms are normally flu-like, but if more severe it can cause dysfunction of the eyes, brain and other vital organs. Sometimes it lies dormant until the person’s immune system is weakened, whence it becomes active. Toxoplasmosis is also a possible cause of a miscarriage or infertility.
One fascinating symptom of toxoplasmosis is psychiatric disorders. Because Toxoplasma gondii can infiltrate the brain, it is known to cause depression or even schizophrenia. As cats are the most common intermediate host, cat owners are more susceptible to toxoplasmosis. Because of this, there is a theory that “crazy old cat ladies” are in fact toxoplasmosis patients.