Understanding the life cycle of a pathogen is crucial knowledge for scientists in order to help them work out how to prevent and control the spread of the disease it causes. This includes the knowledge of things such as;

• Portal of entry

• Persistence in a reservoir

• Portal of exit

• How it is transmitted (mode of transmission)

• Replication

Replication is the process of producing new pathogens from old pathogens. Viruses replicate by taking control of host cell enzymes for replication, bacteria replicate by binary fission (asexually) and fungi/protists may replicate through sexual or asexual means. Persistence refers to the ability for a pathogen to survive for longer periods in reservoirs; its viability outside a host. If pathogen populations cease transmitting, infecting, replicating, persisting or gaining nutrients, or can't survive against the immune system of hosts, then the disease may stop spreading. The three main factors needed for the spread of disease include;

1. A sufficient density of susceptible hosts

2. Growth of a virulent pathogen population

3. Sufficient amounts of transmission

Case Study: Plasmodium Plasmodium has been studied by epidemiologists for decades with the main intention of targeting a stage in the life cycle as a measure of control. Plasmodium causes the disease malaria and requires two types of hosts, being the intermediate host (the human) and the definitive host (Anopheles mosquito). Mosquitoes are a vector for the transmission of the protist, and have been targeted as a control measure. Adult mosquitoes however are highly active and have shown resistance to insecticides, creating difficulty in population control. It is important to target their larvae. Killing both the asexual and sexual forms of the parasite has been made possible, however drug resistance has and continues to evolve. Prevention of transmission via blood feeds through barriers to infection (nets, clothing, closed windows, insect repellent) has reduced the rate of malaria. Travellers to areas where malaria is endemic are encouraged to start chemoprophylaxis prior to travel, a preventative treatment with drugs against malaria. Its actions are to suppress malaria.

How are medications used in disrupting the pathogen life cycle?

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