Inheriting Infections: Can Genes Make You Sick?

Dr. Fred Murphy; Sylvia Whitfield/CDC

Many people are infected with herpes simplex virus type 1, which is responsible for cold sores. Yet only a few carriers develop herpes simplex encephalitis, a potentially fatal brain disease.

It's seems rather obvious that infectious diseases are, well, infectious. But according to a small yet growing number of researchers, your genes also play an important role in determining if an infectious disease will make you sick.

It's hard to imagine a time when we didn't know that germs cause illness. But in the 19th century, it was a mystery why some people developed high fevers and died, says Jean Laurent Casanova, head of the Laboratory of Human Genetics of Infectious Diseases at Rockefeller University in New York City.

"Before infectious diseases were thought to be infectious, there was an opposing theory that proposed that these diseases reflected a specific weakness of families or populations," Casanova.

In other words, people got sick because of something they inherited.

But then Louis Pasteur came along and proved that microbes caused disease, and people forgot about the notion that fevers seemed to run in families. Nearly a century later, the discovery of the first potent antibiotic — penicillin — seemed to confirm that microbes were the culprits. If you can destroy the bacteria causing the infectious disease, you cure the patient.

Genetics Play Role In Infection

But Casanova says the story isn't as simple as that. Beginning in the early 19th century, scientists began to notice that some people could get infected with a disease-causing microbe and not get sick.

For example, nearly everyone is infected with herpes simplex type 1, including most children. But most people get nothing worse than cold sores on their lips. Yet a few wind up in the hospital intensive care unit with herpes simplex encephalitis, a potentially fatal brain disease, says Casanova.

"We know that the microbe is necessary," says Casanova. But what other factors could explain why some people infected with the microbe develop the disease while other people show no symptoms?

Casanova believes that the difference is genetic.

Some people, he says, may have inherited a specific gene that makes them acutely susceptible to a particular infectious disease. In the case of herpes infections, and several bacterial infections, Casanova has found a candidate gene.

Idea Of Genetic Role Gaining Support

Casanova is not the only one who thinks genetic background could explain why some people are susceptible to infectious diseases while others never present symptoms.

"As someone who sees patients, and has seen patients, with serious infectious diseases, I have seen individuals, who are otherwise strappingly healthy in their 20s, come in with overwhelming herpes simplex encephalitis," says immunologist Anthony Fauci, head of the National Institute of Allergy and Infectious Disease.

Fauci agrees that in the absence of any other obvious explanation, someone's genetic background might hold the key. And knowing the genetic defect may suggest drugs in addition to anti-virals or antibiotics that could help certain patients.

More Research Needed

Casanova admits there hasn't been a lot of progress in unraveling the relationship between specific genes and specific diseases. But he says there's a historical reason for that.

"The problem," Fauci explains, "is that the field of human genetics of infectious diseases remains small when compared, for example, to say the field of microbiology," he says.

"There are thousands of laboratories in microbiology searching for novel microbes, searching for the mechanisms for microbial pathogenesis. But there are only a handful number of laboratories investigating the human genetics of infectious diseases."

But that number could be growing. Fauci says Casanova's work is an important reminder that in the case of infectious diseases, it takes two to tango.

"Whenever you have a disease that's caused by a microbe, there are two parties involved. There's the microbe, and there's the host. So you want to learn as much as you can about the microbe, but you also want to learn as much as you can about the host," says Fauci.

Indeed, hosts are as different as the microbes they harbor.