Your body usually does a great job defending you from all kinds of viruses, fungi, and bacteria. However, there are some pathogens out there that can hide from your immune system and stay dormant in your body, waiting for their opportunity to strike.

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Sources:
http://www.ncbi.nlm.nih.gov/books/NBK27090/
Varicella zoster:
http://umm.edu/health/medical/altmed/condition/varicellazoster-virus
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3118253/
Measles virus:
http://www.sciencemag.org/news/2015/01/what-does-measles-actually-do
http://qjmed.oxfordjournals.org/content/108/3/177
http://pmj.bmj.com/content/78/916/63.long#ref-36
Human immunodeficiency virus:
http://www.niaid.nih.gov/topics/hivaids/understanding/howhivcausesaids/pages/howhiv.aspx
https://www.aids.gov/hiv-aids-basics/just-diagnosed-with-hiv-aids/hiv-in-your-body/stages-of-hiv/
http://cid.oxfordjournals.org/content/34/1/91.full
Mycobacterium tuberculosis:
http://www.hindawi.com/journals/jir/2012/139127/
http://www.tbfacts.org/tb/
https://www.niaid.nih.gov/topics/tuberculosis/Understanding/WhatIsTB/pages/detailed.aspx
http://www.ufjf.br/imunologia/files/2009/05/21-de-junho-2012-Revisiting-the-role-of-the-granuloma-in.pdf
Bacillus anthracis:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1769905/
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2784286/
http://www.microbeworld.org/interesting-facts/how-do-they-do-that/microbial-spore-formation
Toxoplasma gondii:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3488815/#CIT0011
http://www.cdc.gov/parasites/toxoplasmosis/biology.html