STDs at molecular level

Pathogen population outweighs human by thousands of times as they easily colonize under any environment. Being invisible to the naked eyes makes us unaware of their stealthy invasion into our bodies. Viruses are particularly mischievous because once infected, you could carry them for life if you have weak immune system or are immuno-compromised, in the worst case HIV currently has no effective cure. Contacts such as sexual one are one of the ugly forms of transmission, of which young adults are most vulnerable because they tend to be highly sexual active. 

As one of the most common Sexually-Transmitted Infections (STIs) in Western countries, genital warts or abnormal tissue growth in the cervix or throat are caused by a highly contagious virus called Human Papilloma Virus (HPVs). HPV is divided into two groups which are low-risk and high-risk types. The low-risk HPVs infect mainly keratinocytes causing benign lesions or genital warts while the high-risk HPVs infect the mucosa of the mouth, throat and anogenital tracts which can cause malignant carcinoma particularly cervical cancer. As condoms or contraceptives are not sufficient for viral protection, women have to be vaccinated with Gardasil or Cervarix but only to get protected against the high-risk types of 16 and 18 which cause about 70% of cervical cancer in women.

The HPV viral life cycle consists of an entry phase, genome replication or amplification phase and then viral assembly and release phase. The icosahedral capsid viruses enter the body via receptors on epithelial basal cells and the identified receptor protein so far is heparan-sulphate proteoglycans. At first, the viral genome is entered as stable episomes (close-circular DNA molecules) of which replication is maintained via the viral proteins E1 and E2, then the viral expression or production of E6 and E7 proteins allows infected cells to expand and produce more virions. To amplify the viral genome, the viruses hijack the cell cycles by disrupting the association between the protein pRb (Retinoblastoma) and a transcription factor E2F and other proteins that involve in cell proliferation. Current treatment is dedicated to the warts only and not the actual viruses, but this helps to reduce their number to ease the immune systems from dealing with too many.

 Human Papilloma Virus under microscope (source: dailymail online): pretty appearance but the symptoms can be nasty

According to medicalnewstoday, 10% of women up to the age of 16 in the UK have carried one or more strains of HPV, 26% of US girls age 14 to 19 have at least one STI. Fortunately, certain genetic variations protect women against cervical cancer and there are no visible symptoms in some cases.

Unlike HPV which causes by virus, Chlamydia is one of the most common bacterial STDs responsible by an agent called Chlamydia trachomatis. The bacteria attach to host cells with its Type III Secretion system (outer membrane protein) to remodel host cell membrane for invasion. Chlamydia is treated with antibiotics but recurrence can occur.

The mouth transmission route is certainly "romance" for pathogens one of which is a Herpes virus family called Epstein-Barr virus (EBV) which causes glandular fever/ infectious mononucleosis or "kissing disease". Immuno-compromised people could progress to cancers such as Hodgkin’s lymphoma, Burkitt’s lymphoma and nasopharyngeal lymphoma, the latter two are prevalent in Africa and East Asia. However, the west is not immune from this disease either, in fact, it is very common in early adulthood, as common as 95% of US citizens and similar figure for the UK. The virus infects specific epithelial cells that lines the mucosa surfaces of the mouth and throat and a type of immune cells called B-cells and once infected, you carry the virus for life. It has its own unique set of latency genes that allow its recurrence through latent infection with manifested symptoms in people with dysfunction immune systems.

On the positive side, STDs can sometimes be misdiagnosed as harmless conditions such as non-contagious Fordyce spots. Despite STD nasty symptoms, it is rare for infection to progress to cancer stage, which depends on environmental influences and individual genetics.