Megadoses of vitamin C (1g every hour during 6 hours, then thrice a day) administered before or after the appearance of cold and flu symptoms relieved and prevented the symptoms by 85%.
Apart from humans and primates, guinea pigs are the only mammals that do not synthesize vitamin C.
Guinea pigs were injected with diphtheria toxin. Those who were on a low vitamin C diet lost more weight than those on a regular diet. Diphtheria toxin depleted vitamin C in the adrenal glands, pancreas and kidneys.
Guinea pigs with limited vitamin C in the diet got injected with sublethal dose of diphtheria toxin. They developed arteriosclerosis in the lungs, liver, spleen and kidneys.
Guinea pigs with low vitamin C reserve, who got injected with lethal dose of diphtheria toxin, died faster than guinea pigs on a regular diet.
Guinea pigs that received large doses of vitamin C, survived even after being injected with several lethal doses of the toxin.
The effect of vitamin C on diphtheria has not been studied since the 40s. in 1971, Klenner reported a case of a girl being cured of diphtheria by an intravenous injection of the vitamin. Two other children, who did not get vitamin C, died. All three also received the antitoxin.
Vitamin C inactivates tetanospasmin in vitro, but not in vivo, in guinea pigs.
30 rats were divided into five groups.
The first group received two minimal lethal doses of tetanus toxin. All rats died.
The second group received a vitamin C injection (1g/kg) together with the toxin injection, and additional injections twice a day. All rats survived. Mild symptoms of tetanus were observed.
The third group got three injections of the vitamin first, and then a toxin with an additional vitamin injection. All rats survived. No symptoms were observed.
The fourth group got the toxin, and after the onset of tetanus symptoms (16-26 hours) they got a vitamin injection. All rats survived.
The fifth group received a toxin, and 40 hours later vitamin C was administered intravenously. All rats survived.
A controlled study of the effects of vitamin C on tetanus in Bangladesh. 117 patients were divided into two groups. The first group received 1g of vitamin C intravenously each day, in addition to the immunoglobulin. The second group only received immunoglobulin.
In the children’s group (1-12 years old), among those who did not receive the vitamin, the mortality rate was 74%. No one died from the group that received vitamin C.
In adults’ group (13-30 years old), among those who did not receive the vitamin, the mortality rate was 68%. 37% of those who did get the vitamin also died. Since the vitamin C dose was the same for both groups, despite the different weight of patients, it is logical to assume that a higher dose of vitamin in the adult group would further reduce the mortality rate.
The authors also report on the results of another interesting study. Since the tetanus toxin and strychnine have a similar mode of action, they decided to check how vitamin C would affect the strychnine intoxication.
60 two-day chicks were divided into four groups.
The first group received 5 μg of strychnine. They showed symptoms of poisoning.
The second group received 5 μg of strychnine, and also 30 mg of vitamin C 30 minutes before that. No symptoms were observed.
The third group received 10 μg of strychnine. They all developed paralysis, and 80% of them died.
The fourth group received 10 μg of strychnine and 30 mg of vitamin C. 20% of them had temporary paralysis, which lasted half an hour. The others showed no symptoms at all.
More about strychnine and vitamin C:  
A very interesting systematic review by Cochrane on the treatment of tetanus with vitamin C. It is worth reading in full, and then comparing in to the systematic review of the subject of aluminum in vaccines.
The authors of this review only found one of the aforementioned studies on the effects of vitamin C on tetanus, in the entire medical literature. However, they report the following, among other things:
Vitamin C shortens the period of common cold. Contrary to the common mantra that the cold left untreated last a week, while when treated it lasts 7 days.
Vitamin C prevents pneumonia.
Cancer patients, who received 10g per day intravenously, lived longer.
Infections and bacterial toxins deplete vitamin C in the adrenal glands. Several experiments proved that vitamin C improves the function of the immune system cells (phagocytes and T-lymphocytes).
Dozens of animal experiments proved that vitamin C increased resistance to infections and bacterial toxins, including the tetanus toxin and other clostridium bacteria toxins (C.perfringens (the one from gas gangrene) and others).
One study determined that there was less vitamin C in the blood of tetanus patients that in the blood of healthy people. And the patients who died of tetanus had vitamin C levels lower than those who survived. Moreover, tetanus patients were observed to have higher levels of dehydroascorbate (the oxidized form of vitamin C), which indicated that tetanus depletes vitamin reserves.
Vitamin C is safe even in very large doses. 100 grams administered intravenously did not cause side effects. When taken orally, large doses of vitamin C could cause diarrhea (more than 30 g/day for sick people, and more than 4-10 g/day for healthy ones, which also indicated that infection depleted vitamin C reserves).
Since this single Bangladeshi study was neither blind, nor randomized, the authors do not recommend using vitamin C in treatment of tetanus, despite the complete absence of side effects. They recommend conducting additional clinical trials. Though no one is rushing to do that, for some reason. Whether WHO or UNICEF are interested in reducing tetanus mortality rate is for you to decide.
In 1936, articles on the effectiveness of vitamin C against pertussis began to appear in the medical literature. The first study was by Otani in Japan, who administered the vitamin intravenously. Later, in 1937, an independent study by Omerod ,  used the oral vitamin C to treat pertussis in Canada. Furthermore, in 1938, vitamin C was successfully used in a study by Vermillion to help cure pertussis in only a few days. It has also been reported that young infants who are breastfed are practically freed of pertussis because of the high vitamin C level in their mother´s milk.
In 1938, a controlled trial found that vitamin C is not more effective than a control substance. However, cod liver oil, belladonna and bromide were used as control substances despite the fact that in an article published in Lancet in 1871 it was shown that whooping cough could be successfully treated using cod liver oil. Even then, scientists have known a lot about choosing a placebo. In the 1950s, several more articles were published on the treatment of pertussis with vitamin C. At that point, however, a vaccine appeared and vitamin C was completely forgotten. As a result, during the past 70 years no one has made further investigations into the benefits of vitamin C against pertussis. Nonetheless, some doctors and parents still successfully use it to treat and prevent whooping cough.
Ayurvedic medicine seems to cure acute and chronic hepatitis B.
Hepatitis B can also be cured with vitamin C. Moringa as well. And desmodium.
Hepatitis B might also be cured with vitamin C. [Baur, 1954], [Kirchmair, 1957], [Calleja, 1960], [Morishige, 1978], [Smith, 1988]
It's possible that Vitamin C supplementation also leads to quick recovery from measles.
A 1952 article analyzes dozens of cases and outbreaks of poliomyelitis, in which paralysis was caused by poisoning with lead, arsenic, mercury, cyanide, pesticides, carbon monoxide, etc. It is also reported that vitamin C, which effectively treats poliomyelitis, has been used to treat poisoning. The author writes that epidemics of pellagra and beriberi were observed earlier, and thus it was believed that these were infectious diseases. Since poliomyelitis was legally recognized as a contagious and infectious disease in 1911, only virologists dealt with it, and ordinary doctors could not participate in the research. It is also the reason why studies to determine whether poisoning could cause poliomyelitis are not being funded.
Полиовирус особенно чувствителен к оксидантам, таким как перекись водорода и марганцовка, и в то же время совсем не чувствителен к таким ядам, как фенол.
Тридцать макак получили укол полиовируса в мозг, с добавкой натурального витамина С, который был любезно предоставлен Merck. Макаки, которые не получили витамина, или получили 0.05 мг, были полностью парализованы вирусом. 0.1-1 мг витамина спасали лишь некоторых макак от паралича, а при дозе в 5-10 мг паралича уже не было ни у кого. Доза в 50 мг была уже менее эффективна, а доза в 100 мг была уже смертельна.
В последующем исследовании выяснилось, что у зараженным полиовирусом макак, которые избежали паралича, уровень витамина С был выше, чем у парализованных.
It was discovered back in the 1930s that vitamin C could help prevent paralysis from poliovirus. Macaques injected with poliovirus that avoided paralysis, had higher levels of vitamin C than the ones that were paralyzed. A 1955 article describes several cases of successful treatment of acute poliomyelitis with large doses of vitamin C (10g every 3 hours). The treatment worked in a couple of days.
Здесь описываются несколько случаев излечения от острого полиомиелита большими дозами витамина С (по 10 г каждые 3 часа). Излечение происходило за несколько дней.
Здесь сообщается, что витамин С инактивирует полиовирус in vitro, а также другие энтеровирусы - Коксаки и эховирус.
A 1949 article reports on 60 patients with poliomyelitis. All of them received 1-2 g of vitamin C intravenously every few hours. They were all healthy in 72 hours. Three patients had a relapse, and their treatment continued for 2 more days. The level of vitamin C in the urine of poliomyelitis patients was lower than in healthy people.
The concentration of vitamin C in the cerebrospinal fluid is 2.5-3 times higher than in the blood. In premature and full-term infants - it is 16 times higher. In premature infants, the concentration of vitamin C is 4-5 times higher than in schoolchildren.
The level of vitamin C in the cerebrospinal fluid in patients with acute bacterial meningitis was significantly reduced.