It’s hard to imagine a time when washing hands between patients wasn’t standard practice for doctors, but in the 1840s, that was the case.

Hungarian pediatrician Ignaz Semmelweis noticed that women who gave birth on the streets were much less likely to contract and die from cholera than women who had their babies in a hospital. Doctors didn’t appreciate his conclusion, but they agreed to take part in his experiment. Washing their hands with chlorinated water between patients brought the death toll way down, and eventually physicians everywhere were expected to wash their hands to avoid spreading disease among their patients.

A portrait of Semmelweis is among the paintings in a new exhibit at the National Museum of Health and Medicine at the Walter Reed Army Medical Center. Paintings in the exhibit are taken entirely from illustrator Bryn Barnard’s 2005 book “Outbreak! Plagues that Changed History.— Like the book, the exhibit takes a look at six world-altering epidemics: bubonic plague, smallpox, tuberculosis, yellow fever, influenza and cholera.

The museum began planning for the exhibit within the past couple of months, spokesman Tim Clarke Jr. said. The timing couldn’t be better, with people everywhere concerned about the H1N1 virus, which the World Health Organization declared a pandemic — an epidemic on a global scale — in June.

“With the pandemic going as it is now, it’s a great opportunity for us to showcase an interest the museum already has had going back a hundred years,— Clarke said.

The current crisis recalls the influenza outbreak during World War I. In his depiction of that epidemic, Barnard turned to San Francisco on Armistice Day in 1918. A citywide ordinance required people to wear masks in public places in an effort to slow influenza’s spread, so one picture shows a couple of law-abiding revelers celebrating the end of World War I by kissing through masks. Others in the background are dancing as they wear masks.

One painting that’s not connected with a particular disease is called “Memento Mori,— which roughly translated means “remember you are mortal.— It follows a custom from the Middle Ages in which people wore jewelry and painted images of human skulls or a skeleton interacting with the living. In Barnard’s take on the concept, though, the skeleton is looking through a microscope, and in the background, a variety of amoeba-like orange shapes show what the skeleton might see there.

The exhibit is fitting for the National Museum of Health and Medicine. Among its permanent exhibits are a look at the different systems of the body and the evolution of the microscope. Formerly known as the Army Medical Museum, it has a special focus on military medicine, with permanent displays taking a look at medicine during the Civil War and battlefield surgery in particular. An exhibit on Trauma Bay II in Balad, Iraq, shows how doctors treated hundreds of patients each month in a tent hospital in southern Iraq until the facility was moved to a base in 2007.

The exhibit will be open until Jan. 22, and Barnard will visit on Dec. 5. The museum is open from 10 a.m. to 5:30 p.m. every day except Dec. 25, and admission is free. Adults need government-issued identification to come onto the Walter Reed campus.