The Viruses That Infect Almost All Humans
A single virus particle – or virion – of SARS-CoV-2 is just 50 to 200 nanometers in diameter. Though diminutive in size, this virus has now upended human life on Earth. Travel is essentially shut down, millions are out of work, and hundreds of thousands are dead.
More biological machines than living entities, viruses number in the nonillions. (There's roughly 10^31 individual viruses, or virions, on the planet.) A virus is composed of an RNA or DNA genome and a protein shell called a capsid. Some even have basic membranes – like outer skin – called envelopes. That's essentially it. A virus' sole drive is to replicate itself, and it can only do so inside living cells, which unfortunately usually results in the death of those cells.
Certain viruses target human cells. A few have grown so adept at invading our cells that they infect the majority of humans on Earth. It's a near certainty that you are, or have been, infected by one of these viruses:
1. Epstein–Barr virus. Spread through saliva, this virus (pictured top) is the primary cause of the mild yet protracted disease mononucleosis, commonly know as mono. In the United States, about 90% of adults show evidence of previous active infection. The term "active" is needed because once the initial infection is beaten back, the Epstein–Barr virus lies dormant in the individual's B cells, a type of white blood cell, for the rest of their life. In this form, the virus is harmless, but it can reactivate when the immune system is stressed and cause illness once again.
2. Human papillomavirus (HPV). Spread through skin-to-skin contact, HPV is the most common sexually-transmitted infection in the world. How common? If you are sexually active, chances are that you will contract it at some point in your life. Nine out of ten infections are asymptomatic and resolve within two years, but some result in warts at the infected area. Worse, a fraction of cases result in cancer of the cervix, vulva, vagina, penis, anus, mouth, or throat. Nearly all cases of cervical cancer can be attributed to HPV. Cervical cancer killed 311,000 people worldwide in 2018. There is now a safe, effective, and affordable vaccine that could drastically drive down HPV infection rates and potentially put an end to cervical cancer.
3. Herpes simplex virus. Ever had a cold sore? Yes? Then chances are good that you've had a herpes simplex virus. There are two types: HSV-1, which causes cold sores, and HSV-2, which causes genital herpes. Both are highly contagious, specifically when the infected individual is symptomatic. Like Epstein-Barr virus, herpes simplex virus can lie dormant inside your body for years after initial infection. According to the World Health Organization, "an estimated 3.7 billion people under age 50 (67%) have HSV-1 infection" and "491 million people aged 15-49 (13%) worldwide have HSV-2 infection."
4. Rhinovirus. You know this virus well: a sore throat, runny nose, congestion, and a cough – must be another common cold. There are more than 160 types of rhinovirus, differentiated by the proteins on their capsid shells, which they use like keys to access cells. They're also tiny – less than a third of the size of the SARS-CoV-2 coronavirus. Almost every human on Earth will be infected with a rhinovirus on many occasions throughout their lives. Despite the ubiquity of rhinovirus, no vaccine has been produced to combat it owing to its diversity. Scientists are ever on the hunt for one, however.
5. Influenza. This virus needs little introduction. Responsible for yearly outbreaks and around three deadly pandemics per century, influenza is highly transmissible and constantly mutating, making it difficult for scientists to provide us with highly effective vaccines. In 2018-2019 alone, the CDC estimates that 35.5 million Americans were sickened, with 490,600 hospitalizations and 34,200 deaths from influenza.