Theory:

Antibiotics:
The bacteria, viruses and so on have many common biological characteristics such as the ability to live inside a host, multiplication etc., When compared to viruses, bacteria share many similar biological processes. Hence, these processes can be blocked mostly by the same chemical or drug.
These drugs that block the biological processes in the microorganisms leading to their death, are known as antibiotics.
The same antibiotics are effective against the organisms of the same group; but they will not work against a microbe belonging to a different group.
 
The antibiotics are substances which commonly block the biochemical pathways that are important for the bacteria. The bacteria protect themselves with the help of their cell walls. The antibiotics such as penicillin block the bacterial processes involved in the formation of cell wall leading to their death. This antibiotic does not affect on human cells as these cells do not have a cell walls. But penicillin affects on most of the bacteria which use the same process in making cell walls. Thus, in a similar way, many antibiotics work against other species of bacteria.
The ineffectiveness of antibiotics on viruses:
Antibiotics are ineffective on viruses as they do not use any biochemical pathways. Instead, they use host cell machinery for making proteins.
 
An example of this is the common cold. On taking antibiotics, the severity or duration of the cold does not reduce. But instead, if an individual is also affected by the bacterial infection along with the viral cold antibiotics can be used as it will work against the bacteria. The most effective way to treat viral infections and disease is vaccination, as it can prevent a person from getting the disease in the first place.
Peptic ulcers:
For a long period of time, it was believed that peptic ulcers which cause acidity (related pain and bleeding) in the stomach and duodenum because of lifestyle habitats. Most of them thought that the stressful life led to a lot of acid secretion in the stomach that eventually led to ulcers.
 
But this was disproved by Barry J. Marshall and J. Robin Warren, who discovered that the bacterium Helicobacter pyloriwere responsible for the peptic ulcers.
 
1024px-Gastric_Ulcer.png
Peptic ulcer
 
Robin Warren, who was born in \(1937\), is a pathologist from Perth, Australia. He saw small curved bacteria in the lower part of the stomach in many patients. He observed that these bacteria always had signs of inflammation around them. Barry Marshall, who was born in \(1951\), was a young clinical fellow who became interested in Warren's findings. Hence, therefore,he succeeded in cultivating the bacteria from these sources.
During the treatment studies, Warren and Marshall showed that the patients with peptic ulcers could be cured only if they were killed off of from the stomach. Thus, they proved by their pioneer discovery that peptic ulcer is no longer a chronic disease and can be cured in a short period of time with the help of antibiotics. They received a Nobel prize for physiology and medicine during the year \(2005\).
Reference:
https://upload.wikimedia.org/wikipedia/commons/thumb/8/8a/Gastric_Ulcer.png/1024px-Gastric_Ulcer.png