Shahar Madjar, MD, Journal columnist

The Diamond Princess Cruise Ship was “a treasure trove of exceptional delight …” until it turned into a devastating experiment in biology.

The ship–1,337 luxurious guest cabins, 18 spacious decks, a state-of-the-art theater, restaurants presenting international cuisine, a Las-Vegas-style casino and a large Izumi Japanese Bath–became a Petri dish infected with Covid-19. At least 712 of the 3,711 passengers and crew members tested positive for the virus, and 14 eventually died of the disease. An outbreak, a mini-epidemic on a cruise ship is an unmatched opportunity to understand the nature of a new infectious disease: the population on the ship is confined and destined to have frequent and close interactions. You mingle. You shake hands. You share a meal, and laugh. And Boom! You get Corona.

The infection rate on the Diamond Princess was 19.2% (close to 1 in 5 passengers) with a 1 in 265 chance of death. Some infected passengers became very sick, but 18% of the infected passengers remained asymptomatic. What determines who gets infected and who is spared, who falls ill and who remains asymptomatic, who lives and who dies?

An infection is a war. Covid-19 is a virus on the attack. The body plays defense. Its army, the immune system, is composed of millions of cells. To win, these cells would have to identify the enemy, recognize it, then attack it viciously. How do you know who your enemy is? You use memory and pattern recognition. The immune system does the same. If it remembers a previous attack, it identifies the enemy and fights back, faster and more forcefully than before. But in the case of Covid-19, a new type of coronavirus, the enemy is unfamiliar. And yet, some people seem to be more resilient than others. Is their immune system more prepared? Does it recognize the new intruder more easily? And if so, how?

Reporting in the August 4th, 2020 issue of the journal Science, Jose Mateus from the La Jolla Institute for Immunology and his colleagues tell the story of a special unit of the immune system, Navy SEALS of a kind (scientists call them CD4+ T cells). How do these specialized cells recognize the enemy, Covid 19, if they hadn’t encountered it before? The researchers isolated these cells from blood samples drawn from individuals that were never exposed to Covid-19 (the blood samples were drawn between 2015 and 2018, prior to the Covid-19 outbreak). They found that these cells react with peptides (small protein molecules) present in both Covid-19 and other types of coronaviruses that typically cause the common cold. Suddenly, it became clear: In some, but not all people, a prior exposure to common-flu coronaviruses trains the immune system in identifying the new enemy, Covid-19. It is as if you were seeing unfamiliar soldiers on the attack, wearing enemy uniforms. You don’t know these particular attackers, but seeing them in their uniform, you know that they are the enemy, and you fight back with all your might!

Reading this you may find comfort. You may, after all, be one of the lucky people who had encountered a common-flu coronavirus, and whose immune system is well-prepared to fight against Covid-19. On the other hand, you can’t really tell. Besides, some of the most devastating outcomes in Covid-19 patients are attributed not to the works of the virus, but to an overreaction of the immune system. In this scenario, the trained immune system identifies the new intruder as an enemy, but fails to completely eliminate it. Then, in a desperate attempt to heal the body, the hyperactive immune system attacks its host, your body. In other words, it suddenly sees soldiers, even its own cells, wearing enemy uniforms everywhere.

Your immune system may be ready, or not. It may react appropriately, or over-react. For now, the best strategy is to try to prevent an infection with Covid-19. Lacking a vaccine, or a treatment that is guaranteed to work, it is safest, for you and the people around you, to wear a mask and to maintain social distancing. We didn’t choose this cruise ship, but we are all traveling on it together.

Editor’s note: Dr. Shahar Madjar is a urologist at Aspirus and the author of “Is Life Too Long? Essays about Life, Death and Other Trivial Matters.” Contact him at smadjar@yahoo.com.