This was the title in the Kalamazoo Gazette above the death notice for the young son of Daniel Harrigan in 1886. Unfortunately, this was neither an isolated case nor would it be the last to befall the family that year. In fact, three in Daniel's family would succumb to the disease in a month's time.
Daniel's four-year-old son died of diphtheria on October 16, 1886. Only a few weeks later, Daniel's eldest child, his daughter Essie, also died of the disease. The newspaper described her as suffering terribly in the afternoon before her death, but noted that Essie's child (a girl of four) was improving. However, about a week after Essie's death her daughter also died “with the same much dreaded and unconquerable contagion.” Within the space of a month, Daniel Harrigan had lost a son, daughter and granddaughter to a disease that can now be prevented with a simple vaccination. I can only imagine the fear and anxiety within the family as first one and then two more loved ones were taken from them.
Between 1876 and 1886 diphtheria was second only to consumption (tuberculosis) in causing mortality in the state of Michigan, with the exception of 1881 when it topped the list, killing 2063.  The first year vital statistics were collected for Michigan (1867) only 110 deaths from diptheria were reported.  The numbers steadily increased so that from 1879 to 1886 (excepting 1881) between 1000 and 1600 people perished from the disease each year, most of them children under ten. [1, 2] According to the Michigan Board of Health Report for 1885, diphtheria had only recently become an epidemic disease in the United States.  This source stated “the first death from diphtheria in this century” occurred in New York City in 1852. In 1885, the mortality rate for diphtheria (percentage of people who perished out of the total number of cases) in Michigan was about 24 percent. 
To truly understand why diphtheria was so feared, besides the obvious reason that it was a killer, it may be helpful to learn more about the disease. Diphtheria is caused by a bacterium that most often infects the nose and throat. According to the NIH “The throat infection causes a gray to black, tough, fiber-like covering, which can block the airways.” In addition to difficulty breathing, symptoms can include a sore throat, fever, chills, a barking cough, bloody/watery drainage from the nose, drooling and a bluish tinge to the skin. [NIH website above] The diphtheria bacterium also produces a toxin which is disseminated throughout the body via the bloodstream, causing damage to organs such as the heart. The disease is spread by respiratory droplets (from a cough or sneeze) of an infected person or asymptomatic carrier or by contact with contaminated objects.
A descendent of Daniel Harrigan recalled that she was told that when diphtheria or some other potentially fatal, contagious disease struck a family that all of the healthy children were sent to family, neighbors or friends. Everyone was willing to do this because they knew they could do the same if their own families became afflicted. The house was quarantined so that only the child, a caretaker (usually the mother) and the father would be allowed in the house. I assume that doctors were also permitted. The mailman, milkman, and other deliverymen would leave their parcels outside the house to decrease their risk of infection.
For the quarantine to take place and be most effective, doctors were supposed to report cases of diphtheria (and presumably other highly contagious diseases) to the city health officer immediately. According to an article in the Kalamazoo Gazette  many physicians took their own sweet time in doing so. The article noted that in one case “the house had been placarded by the city marshal [after reading about the case in the Gazette]. . . before Dr. Mottram received notice of it from the attending physician.”
While today we understand that diphtheria is caused by a bacterium, in 1886, the germ theory of disease was still in its infancy. Only in 1876 did Robert Koch discover that a bacillus caused anthrax. By 1882 he had likewise determined that a different bacillus was the causative agent of tuberculosis. Even so, these were but two cases and many likely didn't believe such a fantastic idea, just as many ideas today are not accepted when first proposed, such as the prion theory of disease [think mad-cow disease]. The 1885 Michigan board of health report alluded to the possibility that diphtheria might be caused by bacteria, but only after pages and pages of quotes from reporting medical officers describing the foul water and fetid air they supposed caused diphtheria in their districts. 
To give you examples of the prevailing ideas on the causes of diphtheria I'll quote from the Kalamazoo Gazette. In late October 1886, one article's sub-headline read “With Falling Leaves Comes Diphtheria – Basement Stenches.”  The author mentioned that “nearly a dozen have died in as many days. None are sick more than a day or two.” He went on to say that most families had at least two sick with the disease which indicated the rapidity of its spread.
The author then discussed the cause of diphtheria as he believed it. “It is the contagion which lurks in the air, in the cesspools, swamps and damp ground that is working havoc. . . By the falling leaves the ground is kept damp and those warm sunny days cause fumes which are poisonous to mankind to arise from the earth. Decaying vegetable matter in the low ground has the same effect. On some it produces diphtheria, on others simply malarial sore throats and others shake with ague. . . Damp and foul cellars are the starting point of much disease.” He recommended sanitizing cellars with carbolic acid or “copperns.” He later stated “much may be avoided by keeping the leaves cleaned up and burned.”  I wonder if that is why when I was growing up everyone still raked their leaves to the curb and burned them rather than using them as mulch for the flower beds. The practice began, as they thought, to reduce infection, but continued even after the initial reason for doing it was long forgotten.
The author continued “there are several localities where the ravages of diphtheria have been more serious than others. Invariably, however, the deaths have occurred in localities where the ground is low and not far from mucky open fields. But one case has occurred in the heart of the city. Why there have not been cases is a conundrum. [Perhaps because his theory is wrong] The east side of north Burdick st. from the drinking fountain to Water st. is in a continuous cess pool from which a stench almost unbearable always arises. The west side of the street is even worse. One cannot pass along Burdick st. at the corner of Water, day or night, without holding his nose.” 
While the number of cases of diphtheria surely would have been lower had our ancestors understood the true cause of the disease and taken preventative measures, without any effective way to treat the disease, mortality likely would not have been much different. No matter what you know, diphtheria is not a disease to be trifled with.
- Twentieth Annual Report Relating to the Registry and Return of Births, Marriages and Deaths in Michigan for the Year 1886 by the Secretary of State of the State of Michigan. 1888. Thorp & Godfrey, State Printers and Binders. Lansing.
- Thirteenth Annual Report of the Secretary of the State Board of Health, of the State of Michigan for the Fiscal Year Ending Sept. 30, 1885. 1886. Thorp & Godfrey, State Printers and Binders. Lansing.
- Kalamazoo Gazette, 9-30-1886, p4
- Kalamazoo Gazette, 10-23-1886, p4