Unless they're eradicated smallpox-style, infectious diseases never disappear. Like an unlucky penny, they can show up at any time.
The reason is because most microbes can survive elsewhere, either in the environment or other animals or both, a concept known as a "reservoir." That is why prevention is the key to public health. And prevention is achieved primarily through practices such as vaccination, water chlorination, pasteurization, sanitation, and good personal hygiene (as well as common sense). If we take away any one of these practices, we can expect relatively rare infectious diseases to come back. Three stories serve to underscore this crucial lesson.
Rabies in Seattle
Recently, a bat was lying on the ground on the University of Washington campus. That is not normal behavior. City folk apparently aren't aware that wild animals -- especially ones that aren't acting normally -- shouldn't be touched. One student learned that the hard way. After he picked up the bat, he got bit and then inexplicably took the bat back to his frat house to show all of his friends.
Country folk won't be surprised to know that the bat had rabies. Thankfully, if it's caught early (as it was in this case), vaccination can prevent the disease from developing, which is otherwise almost universally fatal.
Tuberculosis in San Diego
We think of tuberculosis as a disease of poets and artists in Victorian England, but the disease still claims the lives of 1.7 million people around the world every year, mostly in poorer countries. Still, the developed world isn't immune. For example, several people were potentially exposed to tuberculosis in the San Diego suburb of La Jolla. Of the roughly 9,000 new cases of tuberculosis in the U.S. in 2017, most occurred among immigrants, but the homeless and prison inmates are also susceptible.
Hepatitis A in Nashville
Since December 2017, an outbreak of hepatitis A has sickened 14 people in Nashville. Drug users, men who have sex with men, and the homeless are particularly at risk. Underscoring this fact is a recent outbreak of hepatitis A that occurred mostly among homeless people in San Diego, infecting 600 and killing 20.
We Will See More of This Unless We Address Our Homelessness Crisis
The importance of these three stories is that they serve as a stark reminder that infectious diseases don't just "go away." They are always with us, and some of them are going to pose very big problems if we continue to ignore the growing homelessness crisis in certain American cities.
Homeless camps are ticking time bombs of infectious disease because there is no sanitation or basic hygiene. Diseases like hepatitis A thrive under such conditions. As a result, homeless people are being exposed to diseases that the rest of us never even think about, which is why it is utterly uncompassionate to do nothing and to allow them to continue living in squalor. Cities like Seattle, which are experiencing a surge in homelessness, should brace for a public health catastrophe. That is the natural, biological consequence of allowing people to shoot drugs and defecate in the street.