The army board of which Doctor Reed was president recognized that in Cuba, where they did their work, it would have to be entirely evident that the men experimented upon could not get yellow fever accidentally in Habana, or anywhere else, but only, if at all, in the course of the experiment. They therefore took a piece of unoccupied ground about six miles from Habana and built there a camp of material which could not have been infected with yellow fever. They then got men who had never suffered from yellow fever and placed them in these tents. It was known that if a man exposed to yellow fever were going to have it the disease would develop in less than six days, and if he passed through six days safe and sound he was known to have escaped that particular exposure.

The board therefore argued that if they kept their men in this camp. for a period of two weeks they would be safe from any exposure which might have occurred before their coming to the camp. They also had to provide means which would insure their men not leaving the camp, contracting the disease outside, and thus bringing infection into the camp. This was done by a military guard, who allowed no one to go out or come in without Doctor Reed's permission. Things were now so arranged that if a mosquito was allowed to bite a man, and yellow fever developed, the board could be certain that the yellow fever was due to the bite of the mosquito alone. They did a great deal of experimenting here before they worked out all the details of the way in which the mosquito actually conveys yellow fever. They finally found out that if a female mosquito of one particular species, the stegomyia, was applied to a yellow-fever patient in the first three days of his sickness, and then kept from ten to twenty days and allowed to bite a human being who had never had yellow fever, he would very generally develop the disease within six days after the bite. They also found out that this same man, before he had been bitten by the yellow-fever mosquito, could sleep in the bed in which a patient had died of yellow fever, could be covered with a black vomit from a yellow-fever patient, or be exposed to the emanations from yellow fever in any other way, and as long as he was kept safe from the bite of the mosquito he would not have yellow fever; but this same man, after all this exposure, if afterwards bitten by an infected yellow-fever mosquito, would very certainly catch the disease. I think it would be interesting to give some of the details of the work by which this great discovery was demonstrated.

They had a little frame building built in this camp furnished with jars and the necessary simple material for breeding mosquitoes. The building was carefully screened and guarded, so that mosquitoes could not get in nor out. Eggs of this particular species of mosquito were obtained and hatched in one of the jars. A female mosquito was taken from the brood thus hatched. The male mosquito will not bite, and can readily be distinguished from the female with a magnifying glass by the fact that the male has very heavy feathery feelers (antennæ) growing from his head. The female mosquito selected was put into a small glass tube, stoppered with a little cotton, so that she could get air freely but not escape, taken to Habana, placed on the hand of a patient in the first three days of an attack of yellow fever, and allowed to fill herself with blood. She was then brought back to her former home, placed in a large glass jar, and allowed to digest the blood she had obtained. The jar, covered with a piece of mosquito netting, had

in it a small saucer with a little water, and a lump of white sugar was also provided. Under these conditions the mosquito was furnished with all the necessities of life.

So confident were the men in charge of the mosquitoes that I have known them to put their hands in the jars and let the mosquitoes feed upon them, up to the fifth or sixth day after the mosquito had bitten a yellow-fever patient. The mosquito, you recollect, can not convey the disease till from twelve to twenty days have passed from the time of her biting the yellow-fever case from which she becomes infected. On the other hand, I once saw a party of twelve or fifteen doctors in the mosquito room one day, when the mosquito-bar covering of the jar accidentally came off and the insects escaped into the room. These doctors had come from other countries to investigate the subject, and were not then convinced that the mosquito carried yellow fever. Still, they did not care to put the matter to a practical test in their own persons, and got out of the room so rapidly that the wire-screen door was broken down during their exit. It happened that the mosquitoes in this jar had never bitten a yellow-fever patient and were not infected. After the mosquito had been left in this condition for from ten to twenty days, it was known that her saliva was capable of transmitting the disease. When she was wanted for the purpose of giving somebody yellow fever, a man would take a glass tube, slip his hand under the mosquito netting, put the mouth of the tube over the mosquito, and then fill the mouth of the tube with a cotton stopper, as above described. She would then be taken to the man to whom it was desired to give yellow fever and who had bravely volunteered for the purpose, the cotton stopper taken out, the tube turned upside down with its mouth resting on the skin, and the mosquito allowed to settle. She would then introduce her biting apparatus and slowly fill herself with blood. But before she fills with blood she injects her saliva into the wound, just as does the snake in biting. It is this injection of the saliva that causes the swelling and the burning sensation that is felt at the point where the mosquito bites and which lasts some time after she has finished. The injection of the irritating saliva probably has the effect of making it easier for the mosquito to get blood.

Now, it will be remembered that this man who was bitten had been kept in the camp for two weeks before he was bitten, and isolated in such a way that he could not possibly have contracted yellow fever. There were thirty or forty men in the same camp under exactly the same conditions. Three or four days after he was bitten by the mosquithe developed a well-marked case of yellew fever, although everybody else in the camp remained well. Doctor Reed and his fellow-workers, therefore, very naturally believed that that particular mosquito gave the man yellow fever. They repeated this test twelve or fifteen different times with the same result. Nobody else in the camp had yellow fever. Always within six days after the bite of the mosquito known to be infected, the man experimented upon had yellow fever.

To show how a house could become infected with vellow fever, the board conducted the following experiment: They built a large room and screened it so that mosquitoes could not get in or out. (Whenever I use the word "screen" here I mean the ordinary wire netting or mosquito bar.) The purpose is of course to leave a perfectly free circulation of air, but to have the meshes of the wire or mosquito bar

netting so small that a mosquito can not get through. They then divided this room into two parts by a wire netting extending from top to bottom, so that a mosquito could not pass from one side to the other, but at the same time leaving the circulation of air entirely free, the desire being to show that if it were any miasm, or emanation, or germ floating in the air which caused yellow fever it could freely pass from one side to the other of this netting, and that both rooms, as far as these emanations were concerned, were in the same condition.

Now, to show that the building was uninfected, four men were put in it who had never had yellow fever, two sleeping on each side of the wire netting. They were left there for two weeks and remained perfectly well. Reed then said, "I am now going to infect the room on one side of this wire netting with yellow fever and not infect the other side." He took the two men out of one side and liberated a half dozen infected female stegomyia mosquitoes on this side. The two men still slept and lived on the uninfected side. He then put a volunteer on the side with the mosquitoes and left him there for half an hour, took him out, and within six days this man developed yellow fever, the two men on the other side of the room remaining well.

He therefore argued that, as the two men who had been for so long a time on one side of the wire netting and remained well were breathing the same emanations, and the only difference was that there were mosquitoes on the infected side to which the man had been exposed for half an hour, it was very good proof that the mosquito was the factor which gave yellow fever. He then said, "Now that I have shown you a house infected with yellow fever, I will demonstrate how it can be disinfected and rendered safe." He then caught his half dozen mosquitoes, bottled them up and put them back into their jars, and announced that the building was entirely safe and uninfected, put the two men back into the side which had been infected, and the four continued to sleep and live as safely in these quarters as they had before the infection.

The board had another room built and got all sorts of material infected by yellow-fever patients from the hospital, clothing worn by patients at the time they died of yellow fever, such as mattresses on which they died, soiled in every possible way, pillows and pillowcases saturated with black vomit, and blankets over which basins of black vomit had been poured; in short, material infected by yellow fever in every possible way that could be thought of. All this material was placed in the room, which was made close and tight with very little ventilation, so as to make the conditions most favorable for what was ordinarily considered the best way of insuring the spread of yellow fever. Volunteers who had never had yellow fever were placed in this room, lived and slept there for two weeks at a time, wore this clothing, slept on these mattresses, under these sheets, and yet not a single case of yellow fever was developed from this contact. The men who had undergone this exposure were taken out and kept for two weeks so as to insure that they had not contracted yellow fever from the exposure, and then bitten by infected mosquitoes. They always got yellow fever from the bite of the mosquito, but never in any other way.

At this time the military authorities had had entire control of Habana for about two years. An army doctor had been placed in charge of the health department and given the means and power to do what he thought most likely to free the city from yellow fever.

Yellow fever in Habana was a disease like consumption in Galveston or New Orleans-always there, and always one of the principal causes of death in the city. And this had been the state of affairs as long as anything had been known with any accuracy, either about yellow fever or about the health conditions of Habana; and these things were pretty accurately known for more than a hundred years immediately preceding the time I refer to. When we organized our health department, we believed, as did everybody else, that yellow fever was caused by filth, dirt, and general insanitary conditions, so we went to work doing our very best to correct these conditions. With these efforts Habana very rapidly became a healthy city, as much so as many of our large cities in the United States, but yellow fever did not seem to be affected.

The second year of our control yellow fever was very severe in Habana, but did not attack the native Cuban because he was generally acclimated. Only the foreigner, therefore, was subject to the disease. During the year 1900 many of our prominent American civilians and military officials died of the disease, and the very cleanest and best parts of the city and the people who lived best and took the best care of themselves were most affected. When the army board published their discovery to the world the health department of Habana recognized that it and all the rest of the world had been on the wrong track with regard to yellow fever, and they determined to change their methods and attack the mosquito as the cause of the disease.

They had been convinced by the work of the army board that a human being could only get yellow fever by being bitten by a particular kind of mosquito-the stegomyia-which had previously bitten a man suffering from yellow fever. They therefore arranged that as soon as a man sickened with yellow fever, employees from the department went to the house and screened it with wire netting, so that those mosquitoes that were in the house could not get out and those outside could not get in. A smudge was then made of sulphur, tobacco, or insect powder, as best suited to the circumstances, in the affected house, and in all those immediately around it, with the intention of killing all mosquitoes present. By this method it was hoped that both the mos quitoes that had bitten the man and caused the disease would be killed, and also those that had bitten the man after he was taken sick, and had thus become themselves infected and able to spread the disease. For the purpose of doing this screening a building was arranged very much like a fire station in one of our cities, where wagons, wire screens, carpenters, and men with material for making a smudge, were always kept on duty, who proceeded at once to the place where a yellow fever case was reported to exist.

This method was very successful in its results. After its adoption very few cases occurred where the disease spread from the person infected to others in the neighborhood. It was also determined to destroy as many as possible of the yellow fever mosquitoes in the city. It was known that the female mosquito had to have water on which to lay her eggs, and that these eggs could not hatch without water; that this water had to be very quiet and well protected for the hatching process to take place; that the eggs took about three days to hatch; that after hatching the insect had to live the life of a fish in this water for five or six days. During this fish stage they are known as larvæ, and are well known to everybody in the South, for they are nothing

but the common wigglers always found in standing rain water during the summer months. Now, while in this wiggler stage the insect has to have air, and for this purpose must every little while come to the sur face. At the end of five or six days the wiggler changes into the fullgrown mosquito.

It is known that this particular species of mosquito-the stegomyia, or the yellow fever mosquito-lives and breeds almost altogether in houses and in their immediate neighborhood, and does not leave the house for any great distance. With this knowledge of its life history, the department found it easiest to destroy the mosquito in its wiggler stage, and the most useful means in this direction they found to be the doing away with all the little deposits of water in and near inhabited houses, which the wiggler must have in order to develop into the mosquito. The methods herein described were not settled upon, as might appear from this account, all at once and at the beginning, but many other methods of waging war against the mosquito were tried, found impracticable, and dropped.

With the object of doing away with the breeding places of the yellow fever wiggler all the houses and yards of Habana were carefully examined and all tin cans, empty bottles, and trash of the same kind, which were generally found filled with rain water, and full of yellow fever mosquito larvæ were carefully carted off. Then the necessary openings in all cisterns were covered with mosquito netting, so that the mosquitoes could not get in to lay their eggs. Among the poorer people, who had only barrels and other similar receptacles for rain water (and in Habana every family had something of this kind), the health department arranged these necessary receptacles for them by placing a wooden cover on the barrel, leaving a hole in the center of this cover for the entrance of water, and covering the hole with wire netting, so that mosquitoes could not get in. To enable them to draw off the water without opening the barrel a cheap wooden spigot was placed in the lower part.

Now from the peculiarity of the wiggler, that he has to come to the surface of the water every few seconds to get air, if we put anything on the surface of the water that prevents him getting this air, he drowns just as certainly as a man would who is kept under the water. Ordinary kerosene oil, a tablespoonful or two to a cistern, spreads over the surface of the water and kills the wiggler in this way. He can not break through the scum of oil to get air. But oil very rapidly evaporates and has frequently to be renewed. So oil was only used in Ĥabana where no other method was successful. The privy pits in all the houses there were in the center of the court, covered generally with heavy flagstone. These pits not being in general accessible to the inspectors had to be treated with oil. Once a month, a couple of ounces of oil were poured into the pipes leading to the pits.

To insure that these methods and ordinances were carried out, the city was divided into districts of about a thousand houses each, so that an inspector would get over each district in the course of a month, inspecting at the rate of about thirty houses a day. This inspector had with him two men who used the oil as above described. He had with him printed blanks on which he entered the condition of the premises as to wigglers. These reports were turned in every night to the office of the health department and were consolidated from day to day. At the end of the month we could therefore tell the condition.