The Bloody Red Sun is the result of an atmospheric change caused by a meteorite that hit the Earth and cause the 500 meter tall tsunami that sunk the ship were Satoshi and his friends are and devastate most of Japan and probably the entire world.
Before the "End of the World" as the characters often reffered to the calamity that happen occured, there are many signs already happenning.
Signs in of order of appearance in the manga:
The effects of the bloody red sun to people is varies on each individual and how it affects the human mind is currently unknown. In the early stages of the story it heightened the negative emotions of people that's under severe stress as was shown on Chapter 2 page 19-21 after the Fat Sailorman announced to the ship's remaining survivors about the gravity of their situation the people went into deadly frenzy after they were exposed to the red sunlight as many of them committed suicide while most of them tear the remaining sane sailors apart while some of them tried to rape Tomosaka and Mika as to spend the best last day of their lives.
For people with hidden desires like the Fat Sailorman the feeling was described as "so good" indicating a sense of euphoria while for someone with positive traits such as Tomosaka and Akatani the feeling was described as "not too well" or "strange" indicating anxiety and confusion.
Dr. Akatani theorized that the atmospheric change brought upon by the meteor somehow is the reason why the sun appears to be bloody red and melting. Whatever this "sun" emits causes the human mind, especially the adults to be "broken". However this not the case for Dr. Akatani himself as he was able to maintain his sanity although admitedly he is already feeling the effects that is why he locked Mika in his room to prevent himself from harming her.
Genzou and Amimoto presents another theory. The meteor somehow carried an extraterrestrial virus that is now infecting the remaining population. The sun's appearance itself remains normal however once an adult sees the sun as bloody red that means the person is already infected. Physically there are noticeable change in appearance to people affected such as:
- insane look on their faces
- eyes appears bloody
- abnormal resistance to pain to the point of being impervious
After the ship's remaining survivors was exposed to the red sunlight Tomosaka and Mika describes that the people look "strange" while Satoshi when he find Dr. Akatani near death notice the strange look on his eyes (appears to be bloody). Genzou after being seriously wounded by Amimoto's men is still able to escape and outrun them leading Amimoto to think that he is "infected" too.