They are small, blind and stupid. And they can cave in a whole section of your home without you ever even noticing it by methodically eating away at the structure of your house until a wall or beam buckles. Yes, I'm talking about termites.
Termites are an excellent analogy to what has recently been discovered about many bacteria and viruses in the last ten years. It's possible for us to get "termites" as well and I call them the Silent Killers.
How do these Silent Killers do their dirty work? For the most part, it is through heart, cancer and autoimmune disease. This makes knowledge of these critters critical, because a strong immunity and decreased inflammation can help with in all cases. This is why I recommend that you read my links on How to Boost Your Immunity and How to Decrease Inflammation. Some of the Silent Killers below have vaccines and/or drug therapies that can help as well, but, of course, only if there is awareness.
Below are some of the more significant examples:
1. XMRV. There has been some controversy regarding XMRV, but the general scientific consensus is that Chronic Fatigue Syndrome is likely often caused by this retrovirus.  (Yes, they mean retrovirus as in HIV.) This bad boy actually stands for "xenotropic murine leukemia virus related virus," which isn't a terribly comforting name to squelch public fear about our contaminated blood supply and this potentially deadly health threat.
2. HSV-1 and HSV-2. These herpes simples viruses, associated with cold sores and genital herpes, increase risk for heart disease  and oral cancer , respectively. SOLUTION: One study found that Mercury and other heavy metals were a root cause of recurring herpes (and chlamydia) infections and used Chinese parsley as a treatment protocol.  The reason? Chinese parsley binds to heavy metals so that they can be eliminated from the body. Other plants with evidence of similar properties are cilantro and regular parsely. SOLUTION 2: A recent study found that mugwort components helped with herpes. 
3. HPV. HPV is an STD that can increase risk of oral cancer , anal/penile cancer  and skin cancer.  NOTE: There is currently a vaccine for males that protects against two of the most virulent strains.
4. Periodontitis. As we have mentioned in our link on Natural Ways to Prevent Gum Disease, periodontitis is now considered a risk factor for heart disease.  The reason? It leads to increased Inflammation and inflammation increases endothelial damage and arterial plaque buildup.
5. Mycoplasma. Mycoplasma is the smallest pathogen and has no cell wall. It has several variants, including Mycoplasma pneumonia, which cause walking pneumonia and M. hominus and Ureaplasma urealyticum, which are quasi-STD's. Mycoplasmas are nasty and can hide deep within cells, such as white blood cells, making them at time notoriously hard-to-detect. This is such an important topic that we cover it in a separate link called Ten Ways Undetected Mycoplasma Will Kill You, but suffice it to say that it likely contributes to prostate cancer, certain arthritises, chronic fatigue, male infertility and a host of other issues that we do not know or understand yet. NOTE: In women, mycoplasmas can lead to PID (Pelvic Inflammatory Disease) and lupus.
6. SV-40. This monkey virus was accidently given to millions of us humans in polio vaccines and some researchers feel that "SV40 in humans is associated with inflammatory kidney diseases and with specific tumor types: mesothelioma, lymphoma, brain, and bone."  NOTE: Some researchers feel SV-40 is only a problem for monkeys and does not cause cancer-related issues in humans.
7. Epstein-Barr. This herpes virus is best known for causing Mono, i.e. mononecleosis. It turns out this ubiquitous bug is a powerful trigger for many autoimmune diseases, including multiple sclerosis , lupus , arthritis  and possibly Type I diabetes.  It is also linked to various cancers. Almost everyone (about 90% of the population) is infected and there are no direct treatments. See the comments at the bottom, though, for some exciting research breakthroughs.
8. Coxsackie Virus. This virus generally produces mild symptoms but in some inflames the heart, a condition called myocarditis. This can be debilitating for some and even life-threatening for others. There is no known cure. 
All of this underscores the need to Maximize Your Immunity. Scientists recently found that when they blocked the ATM and CK2 kinases (enzymes) that cell growth grew at a very rapid rate, something linked to various cancers. What increases these same kinases? I3C, the famous phytochemical in broccoli and other cruciferous vegetables, activates these two and may explain some of its cancer-fighting powers. 
2) Circulation, 2000;102:2335, "Chlamydia pneumoniae, Herpes Simplex Virus Type 1, and Cytomegalovirus and Incident Myocardial Infarction and Coronary Heart Disease Death in Older Adults"
3) Arch Intern Med. 2009, 169(13):1233-1240, "A Pooled Analysis of the Effect of Condoms in Preventing HSV-2 Acquisition"
6) Journal of the National Cancer Institute, 2006, 98(6):389-395, "Human Papillomavirus Infection and Incidence of Squamous Cell and Basal Cell Carcinomas of the Skin"
8) Virology, Jan 2004, 318(1):1-9, "Simian virus 40 infection in humans and association with human diseases: results and hypotheses"
9) ANN NEUROL, 2010;67:824 830, "Primary infection with the Epstein-Barr virus and risk of multiple sclerosis"
10) Journal of Clinical Investigation, Dec 1997, 100(12):3019-3026, "An increased prevalence of Epstein-Barr virus infection in young patients suggests a possible etiology for systemic lupus erythematosis"
11) Journal of Clinical Invest, 1981 April, 67(4):1134 1140, "Elevated Levels of Antibodies to Epstein-Barr Virus Antigens in Sera and Synovial Fluids of Patients with Rheumatoid Arthritis"
12) Annals of the New York Academy of Sciences, April 2002, 958:7-25, "IMMUNOLOGY OF DIABETES: AUTOIMMUNE MECHANISMS AND THE PREVENTION AND CURE OF TYPE 1 DIABETES"
13) Circulation, 1999, 99:1091-1100, "From Myocarditis to Cardiomyopathy: Mechanisms of Inflammation and Cell Death"
14) Cell Host & Microbe, Dec 2010, 8(6):510-522, "An ATM/Chk2-Mediated DNA Damage-Responsive Signaling Pathway Suppresses Epstein-Barr Virus Transformation of Primary Human B Cells"
15) Cancer Prev Res, Jul 2010, 3:818, "A Novel Mechanism of Indole-3-Carbinol Effects on Breast Carcinogenesis Involves Induction of Cdc25A Degradation"
16) Acupunct Electrother Res, 1995 Aug-Dec, 20(3-4):195-229, "Role of mercury (Hg) in resistant infections & effective treatment of Chlamydia trachomatis and Herpes family viral infections (and potential treatment for cancer) by removing localized Hg deposits with Chinese parsley and delivering effective antibiotics using various drug uptake enhancement methods"