Can a parasite kill you if you do not treat it or get rid of it?
The answer to this question depends on the parasitic species involved. However, as parasites usually need their host to reproduce and survive, it is not in their best interest to kill their host quickly. As such, with respect to some exceptions, parasites do not usually kill their host right away and, for some species, it could take years for the host to die from its parasitic infection.
The exception to this general rule comes from the extremely rare infections of the brain and the central nervous system caused by some free-living amoebae where death of the host happens quickly in more than 97 % of the cases, even with a treatment. This type of diseases is called granulomatous amoebic encephalitis and it is caused by Acanthamoeba species or Balamuthia mandrillaris and primary amoebic meningoencephalitis when it is caused by Naegleria fowleri. For these diseases, death of the host occurs quite quickly, within weeks for the granulomatous amoebic encephalitis and within days for the primary amoebic meningoencephalitis. However, a total of approximately 1000 cases of both these diseases have been reported worldwide so far. The lack of effective treatment against these parasites is directly linked with this very few number of cases. As mentioned earlier, it is not very logical for a parasite to kill its host quickly, as it usually needs it to reproduce and successfully fulfill its life cycle. However, these deadly parasites are free-living amoebae and are though facultative parasites infecting humans only accidentally, which means that they do not need a host to fulfill their life cycle.
At the other end of the spectrum, a lot of parasites are able to infect people silently without causing any symptoms. Furthermore, a good proportion of the known human parasites would not kill their host, even though they are able to make it sick. Some parasites are also not fatal and usually able to go away by their own without any treatment. It is the case for example of infections with Giardia or with one of the coccidian parasites, namely Cyclospora cayetanensis, Cryptosporidium species and Cystoisospora belli occurring in an otherwise healthy individual.
Moreover, the majority of parasitic diseases caused by helminths are not considered to be fatal. However, it is always possible to suffer from life-threatening complications while having a parasitic disease, even though otherwise healthy. It is also interesting to note that some parasites that are not known to kill their otherwise healthy host can nevertheless be able to kill hosts that have a compromised immune system or other contributing risk factors such as being very young or old or suffering from an active illness. People in developing countries are also more at risk to die from parasitic diseases, as host’s related factors, such as malnutrition, poverty and potential co-infections with other types of microorganisms, make them more susceptible to die from parasitic diseases. Furthermore, the majority of the cases of parasitic diseases in developing countries involved young children considered more at risk to die from the infection.
Between these extremes, there are effectively some parasites that would eventually kill their host when not treated. However, it is usually a long process that can take years. For example, the species of Leishmania responsible for the visceral form of the disease and the species of Trypanosoma responsible for the African sleeping sickness will eventually kill their host most of the time if the disease is left untreated.
Finally, even though some parasitic diseases can be harmless, it is always better to get a proper diagnosis in order to receive an effective treatment when it is required. Treating the parasitic infection will greatly reduce the possibility of having a further life-threatening complication.
Source: Centers for Disease Control and Prevention