Medications which are used to treat infectious diseases come in the form of antimicrobial agents. The type of antimicrobial is entirely dependent on the type of organism causing the infection, whether it be a bacterium, virus, fungus or a protist. There are four classes of antimicrobial agents for these pathogen types.

1. Antibiotics

2. Antivirals

3. Antifungals

4. Antiprotozoal

What are antibiotics? Antibiotics are antimicrobial chemicals which act to inhibit or destroy bacteria, as they target structures and/or processes which are only present in bacteria. As bacteria are prokaryotes, it has been relatively easy to find and develop antibacterial drugs that have minimal side effects of a number of other organisms. Antibiotics that are classified as penicillins and cephalosporins all interfere with the synthesis of the peptidoglycan layer in prokaryotic cell walls, which does not affect eukaryotes as they have neither the peptidoglycan components nor the enzymes to synthesise them. A second class of drugs including chloramphenicol, tetracyclines and erythromycin bind to prokaryotic ribosomes and inhibit protein synthesis. Prokaryotic ribosomes are different to eukaryotic ribosomes in terms of their structure, hence these drugs having minimal effect on eukaryotic cells. Antibiotics have been both overused and misused. Misuse of antibiotics commonly occurs when prescribed antibiotics have not been taken for the full course. Other times, patients habve been prescribed too early or as a preventative measure. As a result, antibiotic resistant bacteria have rapdily evolved and even resulted in some "superbugs".

What are antivirals? Antivirals are antimicrobial chemicals which inhibit the ability for viruses to replicate. If fewer viruses are made, this reduces the duration and spread of the disease. Antivirals treat viral infections through symptom minimisation and infectivity, and shortening the duration of illness. Designing safe and effective antivirals is difficult as viruses use the host's cells to replicate. This poses a problem to find targets for the drug that will interfere with the virus without harming the host organism's cells. The major difficulty in developing vaccines and antiviral drugs is related to the high degree of variation in viruses; new strains develop rapidly, making previous vaccines obsolete. Various points in the life cycle of a virus can be targets for antivirals. Examples of antivirals acting on the various target points include antivirals which:

1. Inhibit binding or attachment (aka 'entry blocking' drugs)

2. Inhibit entry or penetration via blocking protein channels in the cell membrane of the host

3. Inhibit transcription of the virus by blocking transcription factors to viral DNA

4. Prevent the release of the newly assembled viruses from the cell membrane