General Virology

Viruses are the smallest form of life that are capable of replications. Unlike bacteria they do not have their own replication mechanisms, yet they are capable of using the machinery systems of other organisms to replicate. Viruses can infect bacteria, plants, or animals cells (including humans).  As viruses require receptor on the surface of the cells that they infect for attachment, most viruses are capable of infecting only specific organisms. For example, Avian influenza can only infect birds. Human are only infected with a type of influenza virus that can attach (on the receptor) to human respiratory cells. For avian influenza virus to successfully infects the human population, there should be changes at molecular level that will enable them infect human cells. There are two known mechanisms that provide the necessary changes to cross this inter-species barrier. In practice, these mechanisms are only important in Influenza virus infection and possibly in Rotavirus.

There are a number of viruses that causes mild or asymptomatic infection in humans. Only a few viruses are associated with serious treatable and non-treatable illnesses. 

There are a number of methods to classify viruses. The one widely used by Virologist is the Baltimore Classification of Viruses.

Characteristics of viruses

?         Viruses are the smallest organisms in our planet.  

?         They have either DNA or RNA as a genetic code.

?         They require living cell for their replication.

Click here if you are interested to know about Classification of Viruses and the strategies of Viral replications

Vaccines are available for a number of viral infections. They are used to induce immunity to specific viruses and one exceptional vaccine used to induce immunity to a different viral agent is Variolla vaccine. It is used to induce immunity to Smallpox and it was successfully used to eradicate the disease from the globe. At present the vaccines on use contain antigens that stimulate the immune system to produce cell mediated or humeral (antibody) immunity. But for the future there are candidate vaccines that constitute genetic codes for the desired antigens and they are expected to have the advantage of stability during transportations and storage. The current vaccines are made of live attenuated virus or from viral components.  Live vaccine have the advantage to stimulate cell mediated immunity and antibody production, therefore they are more effective. However, they have disadvantages in that they may cause diseases in immunocompromised patients. On the other hand, killed vaccine or peptide vaccine can stimulate mainly the humeral system (antibody), and they are short-lived and less effective. The latter vaccines has better safety records as they cause less adverse effects.

In General most vaccines are safe. Some may cause mild illness. Skin rash and inflammation at injection site are the common adverse effects. The claim that MMR can cause autism is unfounded, and vaccinating children with this vaccine is recommended and it is one of the successful methods of controlling or eradicating Measles and/or Mumps.

Click here if you are interested to know about vaccinations.

In patients who are considered to have a high risk of developing serious illness or complications when they contract certain viral infections, the use of antibodies have the advantage to attenuate or prevent infections or their complications. This method of transferring immunity to a susceptible host is known as Passive Immunity.  There are theoretical risks on the use of immunoglobulin from serum. Now a days there are a few number of licensed humanised immunoglobulins. These immunoglobulins are synthesised in a laboratory with the help of hybridoma technology. At present these immunoglobulins are expensive and not widely available. The immunoglobulins extracted from human serum still on use and clinicians when they prescribe them must weigh the theoretical risk against the benefit they provide on individual basis.

Click here if you are interested to know more about the immunoglobulin.

Antivirals are drugs that are used to treat viral infections. There are over 30 antivirals on use. They are most effective if they are used at the early stage of infection. Nevertheless for most chronic infections like HIV or hepatitis B or C, treatments are recommended only once chronic infections are established and certain criteria are met. In recent studies where therapy was started in the acute stage of hepatitis C infections, the response rate was very high compared to the response rate seen in chronically infected patients. Some experts believe that treatment for hepatitis C infection should be started as soon as possible if the patients are known to be infected recently. On the other hand, this strategy may not be useful for hepatitis B or HIV infections. Some of the drugs that are used to treat HIV infections are associated with serious side effects, and most experts believe that the disadvantages associated with side effects outweigh the advantages of controlling HIV replications at the early stage of infection.

Click here if you are interested to know more about antivirals.