When the United States began vaccinating against the measles virus in the 1960s, children fortunately stopped getting measles. But that wasn’t the only thing; suddenly, American children had plummeting risks of dying from all sorts of other infectious diseases, too. The same thing happened in European countries joining the effort. But how can a vaccine protect against infections that it’s not even targeting?
Like all other microbes that infect us, the measles virus is not a big fan of our immune system. Cells of the immune system are constantly on the lookout for invaders and will spring into action if an uninvited guest is discovered. Viruses like the measles virus fight back by hiding, by trying to trick the immune system and sometimes also by counterattack. This war between our immune system and various microbes is ongoing throughout our lives. It’s happening inside you at this very moment.
Pathogens have evolved various weapons to target the immune system, but the measles virus has found a particularly effective one. It can cause something you can think of as immune memory loss. Usually, certain cells of the immune system retain a memory of previous adversaries. This is clever, because it decreases the time it takes for the immune system to react if it encounters the same enemy again. Then, there will already be a tried-and-tested battleplan ready for deployment to rob the infection of the chance to take hold. This immune ‘memory’ is the reason vaccines can protect against developing a disease, and also the reason you only get diseases such as chickenpox once in a lifetime.
When the measles virus causes ‘memory loss’ in our immune system, though, all this valuable information is lost. This benefits the measles virus itself, but it’s also a boon for all sorts of other bacteria and viruses. Suddenly, these pathogens have a much easier time infecting us. Therefore, infection with the measles virus predisposes you to all sorts of other infections, too. In fact, it is estimated that the measles virus used to contribute to half of childhood deaths from other infections.
Such one-two punches are quite common in the world of infections. A straight right from an initial infection and then a left hook from a second one that exploits the chaos to its own benefit. On the one hand, this principle illustrates why vaccines were (and still are) the uncrowned king of medical science. But it is also bad news, because there are still plenty of dangerous microbes that we don’t have vaccines against yet.
A particularly good example is HIV, the virus that causes AIDS. HIV attacks certain cells of the immune system called T-cells. You can think of T-cells as the generals of the immune system, because they are responsible for orchestrating your immune responses. When HIV attacks T-cells, they eventually succumb to the virus. This means the immune system becomes weaker and weaker, and eventually it cannot keep up with all sorts of other microbes. As a result, HIV-infected people become vulnerable to otherwise harmless infections. Microbes, which normally live in or on us in peaceful coexistence, sense an opportunity and begin to grow out of control. The relatively harmless fungus Candida albicans – which lives on over half of us – can turn into a serious infection. Herpes virus 8 can go from being relatively harmless to causing a form of cancer called Kaposi’s sarcoma. Even the flu can become deadly.
The infectious burden of HIV is taxing on the body, and even though we now have anti-HIV drugs that help patients live much longer than previously, they still die earlier than non-infected people. They also have an increased risk of everything from cancer to cardiovascular diseases. And in fact, it turns out that HIV infection in itself increases the rate of biological ageing. Patients with HIV are five to seven years older biologically than their actual age as measured by the epigenetic clock.
As a whole, there are countless viruses that target humans, including ones we don’t know about yet. It’s not hard to imagine that some of these contribute to ageing or disease; nor is it difficult to envision that diseases, for which we haven’t yet identified the cause, could turn out to have bacterial or viral involvement. Okay, it might not be particularly wise to become a paranoid hypochondriac either, but it’s certainly worthwhile using a little common sense and of course, getting vaccinated.
Nicklas Brendborg is a PhD student of molecular biology at the University of Copenhagen. In 2015, Brendborg published his first book, Top Student. He also co-authored Lars Tvede’s bestseller Supertrends.