Not only Karl May's eyesight was affected in his early childhood. His general health suffered as well: "... [I was] for full four years in bad health" . After treatment in Dresden he "returned, also for the rest recovered, home."  May quite clearly states that his general health suffered apart from the eyesight.
The only other remark about his childhood condition Karl May made in the following passage : "I was born as a sick, weak child, who even at the age of six years crawled on the ground, without being able to stand or even run ... I was blind three times ... ". This of course is a literary interpretation that nevertheless was based on Karl May's personal experience. 
There are no more clues left by Karl May, except perhaps in a book found in his library: "Der Saugling, Seine Ernaehrung in gesunden und Kranken Tagen", (The Toddler and its Nourishment in Health and Sickness) published in Hamburg in 1888 . It is quite understandable that Karl May might have tried to learn about his childhood illness from a contemporary text. We know that he did attempt to find out more about his mental state he was in when young from a textbook of psychiatry, also discovered in his library . Some leads to the childhood sickness of Karl May have been left by people who observed him. We also have photographs of Karl May that we can study.
When May visited Komotau (the town of Chomutov presently in the Czech Republic) on the 13 July 1897, there is a record of his appearance at Hotel Scherber : " [The married couple entered the hall] ... She was an impressive womanly figure, as one would imagine a Wagner's Valkyrie, however the look of Karl May disappointed us. We imagined a hero's stature, instead of that in came a hardly middle sized man, with somewhat bow legs (from to much horse riding, as Fischer explained) and with upstanding, mostly uncombed hair, a gold pince-nez on his nose..." 
W. Raddatz mentions in his book that Karl May did not like his small stature. Only in his older years May felt better, he says, evidently because of his magnificent characteristic head, which was often praised. Karl May himself gives his height as 166 centimeter in an article about himself . His two main sidekick heroes, Hadji Halef Omar ("Halef was a curious little fellow. He was so small..." ) and Sam Hawkins ("this small fellow ... [with] Two thin, crooked legs..." ) are both presented as short persons.
In December 1862 Karl May was found unfit for the compulsory military service. Until now it was thought this was because of Karl May's poor eyesight. Yet May's vision did not seem to represent a problem during his life. Perhaps there was another reason, more in connection with the aftereffects of childhood rickets.
On the photographs of Karl May, in particular those from his younger years, we can recognize a broad forehead and largish head [Photo 1 - KM in 1896].
On another picture [Photo 2 - KM as Kara ben Nemsi] we see his short stature and a hint of bowlegs.
There is one malady that was so widespread in the 19th century as not to attract any special attention. It was the rickets.
The first description of rickets comes from the year 1645 . Elucidating the real cause of rickets was a long time coming. The ultimate cause - lack of vitamin D - was finally confirmed in 1921. Only in the mid 1930's it was shown vitamin D could be synthesized in the skin as well as occurring in food.
Rickets became known as "the English disease" that was due, as we now know, to the lack of exposure to the ultraviolet rays in that country. In the Victorian era the sun had difficulty piercing the enveloping smog of the cities during the times of industrialization. City dwellers and those living close to the developing textile industry were particularly affected. Deprivation of sunlight rather than faulty nutrition was thought to be a major factor, although they did not know the mechanism. The disease was so common that in an 1884 survey in Glasgow, every child examined had some signs of the condition . A report from the middle European city of Prague stated in 1863 that 31 children out of 100 treated at a polyclinic suffered from rickets .
Rickets is the result of deficiency of vitamin D in children. Infants in their first year are susceptible to rickets because of the low content of vitamin D in both human and animal milk, and if, for various social and cultural reasons, their mothers wrap them up in clothes which prevent their exposure to sunlight. Rickets start around the third month of life, most commonly in the second half of the first year. The infant with rickets has often received sufficient calories and may appear well nourished, but is restless, fretful and pale, with flabby and toneless muscles. Gastrointestinal upsets with diarrhea are common. The infant is prone to respiratory infections. There is failure to sit up, stand, crawl and walk at the normal ages.
The bony changes are the most characteristic and easily identifiable signs of rickets. As we have never seen Karl May's body we do not know whether there were signs as the 'beading' of the ribs or 'rickety rosary',or the prominence of the sternum ('pigeon chest'). If the condition still persists after six months of life, there later appears the 'bossing' of the frontal and parietal bones and delayed closure of the anterior fontanel on the skull. If rickets continues into the second year of life, these signs may persist or be magnified. Deformities such as kyphosis of the spine develop as a result of the new gravitational and muscular strains, caused by sitting and crawling. When the rachitic child begins to walk, deformities of the shafts of the leg bones develop, so that 'knock knees' or 'bow legs' are added to the clinical picture. Diminution in stature is due to the following reasons: The actual growth of bone is retarded, especially that of the tibia and femur, which may be as much as one-quarter shorter than those of a normal child of equal size and age. Secondly because of bending of the bones of the legs, and finally because of spinal deformities.
We do not know whether Karl's mother or the grandmother insisted on keeping the baby wrapped in swaddling clothes during the first year of his life. It is highly probable that later on the child had been kept out of sunshine on account of the sick eyes. Because of the shape of Karl May's head we may assume that the fontanels stayed long open, until the fourth year of his life (usually fontanels close at 16 months). If the closure of the fontanels is delayed as the skull develops it becomes broader and flatter than normal. It used to be called 'caput quadratum', and the prominent shape of the front, the 'bossing', is sometimes called 'frons quadrata'. [Photos 1 - 3 - 4]
Karl May was absolutely correct when he stated in his biography : "That I shortly after my birth became seriously ill, lost eyesight and was for full four years in bad health, was not the consequence of heredity, but of purely local conditions, the poverty, foolishness and the ruinous medicinal quackery to which I fell victim. As soon as I came into the hands of competent physicians, the eyesight returned to me and I became an extremely strong and resilient youngster..." 
Nowadays the basic prophylactic measures to prevent rickets are well known. If a young child can have one pint of cow's milk, an egg and some butter daily, and has free access to a garden in which he/she can run about in the sun, the administration of a supplement of vitamin D is probably not essential for its future health. Vitamin D is found in fish liver oils, egg yolk and milk. Butter, eggs and fatty fish such as herring, mackerel and salmon contain vitamin D. Vitamin D is also manufactured in the skin in the presence of sunlight. From this simple rule we can deduce that it was not so in baby Karl's case. There was little milk available and little Karl was kept away from the sunshine because of his sore eyes. Rickets, a deficiency disease in lacking vitamin D (and as we know Karl was also deficient in vitamin A with resulting xerophthalmia), was the unfortunate outcome.
Since the 18th century it was known that cod-liver oil was of help in rickets. Children who could not take cod liver oil were prescribed halibut-liver oil. This would have been the correct treatment for little Karl, since the concentration of vitamins A and D is three to four times that of cod-liver oil.
After four years of such deficient diet and lack of sunshine, Professors Grense and Haase at Dresden saw the little boy. What these gentlemen did was that they had a talk with the mother and prescribed cod-liver oil or halibut-liver oil. The result had been a marked improvement in the boy's health. Severe rickets can be arrested, but the bony deformities are lifelong. The severity of rickets depends on the duration and extent of nutritional deprivation. If the child has developed bowlegs, raised bony bumps along the ribs (rachitic rosaries), frontal bossing of the skull, and other deformities of the bones, these become permanent.
The resilience and stamina of Karl May as a person have to be admired. Against all the odds in his life working to his disadvantage Karl May became an exceptionally creative writer. At present rickets, a deforming bone disease, the results from too little vitamin D in the diet or lack of sunlight, as well as xerophthalmia, lack of vitamin A, are treatable diseases that had disappeared in the developed world.
All the above is truly portrayed in the caricature of Karl May:
Please click on the hyperlinked reference numbers to return to your place in the text.
 Karl May: "Ich." Karl-May-Verlag, Bamberg, 1968, p.40.
 Karl May: "Ich." Karl-May-Verlag, Bamberg, 1968, p.43.
 Karl May: Old Surehand, Band 1, Freiburg 1894, pp.411-412.
 Harder, R.: Karl May auf amourösen Pfaden? - In KM Bibliothek as: It.KM.Jahrbuch 31, 286.
 Dr.W.Griesinger: "Pathologie und Therapie der psychischen Krankheiten." Braunschweig 1871.
 Griese, V.: "Karl May - Stationes eines Lebens - Eine Chronologie seiner Reisen." - Sonderheft der Harl - May Gesellschaft Nr.104/1955; entry for the 13th July 1897.
 Raddatz, W.: "Abenteuerliche Leben Karl Mays." - Sigbert Mohn Verlag, Germany 1965, pp.69-70.
 Karl May: "Freuden und Leiden eins Vielgelesenen" Deutscher Hausschatz No.2, (Oct. 1896 - Oct 1897; p.312).
 Karl May: "In the Desert." - The Seabury Press, New York 1977, p.1.
 Karl May: "Winnetou." - The Seabury Press, New York 1977, p.1.
 Glisson, Francis: "A Treatise of the Rickets, Being a Diseas Common To Children", London 1651.
 Leavesley, J.: "Shedding some light on rickets" - Australian Doctor, 6 November, 1998, p.80.
 Thomayer, J.: "Pathologie a Therapie Nemoci Vnitrnich" - Nakl. Bursik and Kohout, Praha 1897, p.374.
 "Davidson's Principles and Practice of Medicine" - Churchill Livingstone, London 1987, pp.64-66.
 Karl May: "Ich." Karl-May-Verlag, Bamberg, 1968, p.40.