Statement 1 states that antibiotics are used to destroy disease-causing bacteria, which is true. Antibiotics are medications specifically designed to target bacteria and kill or inhibit their growth. This is an important way to treat bacterial infections.

Statement 2 states that antibiotics can be applied to the skin, swallowed, or injected to fight micro-organisms inside the body. This is also true. Antibiotics come in various forms such as creams, pills, or injections, allowing them to be used in different ways depending on the type and location of the infection.

Statement 3 states that antibiotics are effective against disease-causing viruses. This statement is incorrect. Antibiotics are designed to target bacteria, not viruses. Viral infections, such as the common cold or flu, do not respond to antibiotics. Treating viral infections requires different types of medications, such as antiviral drugs.

Statement 4 states that tetracycline was the first antibiotic to be discovered. This statement is incorrect. The first antibiotic to be discovered was penicillin, which was found by Alexander Fleming in 1928.

Therefore, only statements 1 and 2 are correct.