1.2 Chronic and latent infections
So what is the key difference between acute and chronic viral infections? Essentially, they have different reproductive strategies. After infection, viruses such as SARS-CoV2 and influenza replicate quickly and then spread by aerosol droplets, which are inhaled into the nose, throat and respiratory system – they complete their life cycle within days, before the immune system has fully geared up to eliminate them.
Conversely, chronic infections such as Herpes simplex and HIV can evade the immune response for years. During this time, the virus is shed more slowly, sometimes sporadically and the method of transmission is often by direct contact between individuals.
These persistent viruses have a variety of strategies for evading the immune response. Here are some examples:
- HIV mutates continuously within an individual to avoid being recognised by antibodies.
- HIV subverts the cellular machinery which promotes recognition of virus-infected cells.
- Herpes simplex remains latent in neurons, and deploys decoy proteins that fool the immune system into recognising the cell as not-infected.
- Epstein Barr virus (EBV) secretes a signalling molecule (cytokine), which deviates the immune response away from that which eliminates EBV-infected cells.
- Human papilloma virus (HPV) (Figure 1) produces very low levels of viral proteins, so the immune system has little foreign material to recognise.
Indeed, each of these viruses has numerous systems for evading immune responses. It is a fascinating area of research. However, to understand it requires a knowledge of how the immune system would normally eliminate a virus or a virus-infected cell. Before that you need to understand how the immune system recognises viruses and that is based on some knowledge of virus structure.