Re: Would it ever be necessary to find the 'patient zero' of a virus?

For most viruses that enter humans from a non-human natural host, "patient zero" is the only patient infected. For example, with the H5N1 avian influenza virus that has now infected many humans, each human was a different "patient zero" and did not pass the virus on to other humans to create any human epidemic or even smeall "outbreak". This situation holds true for rabies viruses and most other viruses that humans get from animals, they can harm the infected human but the infected human cannot pass the virus on to more people.

The most common reason why it is necessary to find "patient zero" in outbreaks, where a pathogen (be it virus, bacteria, fungus, head lice, or any other type of transmissible organism) is not for vaccine design but for epidemiological control. For a great many different types of infections, most infected individuals do not contribute much to the outbreak while a few individuals, sometimes referred to as "superspreaders" are resonsible for infecting the majority of cases. In the case of a foodborne bacterial outbreak, the "superspreader" is someone who works in a resteraunt or a bakery or something like that. In a sexually transmitted disease outbreak the superspreader might be a prostitute. In an influenza outbreak a superspreader could be anyone coughing and sneezing on an airplane or train station.

There are dozens of famous and exciting (at least to epidemiologists) cases of tracking down "patient zero" for many types of outbreaks. One of the most famous cases is "Typhoid Mary". It turns out that she didn't infect hundreds or thousands of people, but she did infect quite a few, although she herself never developed typhoid fever symptoms. In other cases, the source of an outbreak is not a person, but a batch of food, such as a shipment of strawberries from a field that was irrigated with contaminated water, or a crock pot full of stew in a cafeteria. A famous case of a deliberate outbreak happened in 1984, when followers of Bhagwan Shri Rashneesh sprinkled homegrown salmonella bacteria on supermarket produce, door handles, and restaurant salad bars in Oregon. Nobody died, but 751 people became ill. The poisonings were preparation for attacks meant to keep voters home during a local election in which a cult member was running for a county judgeship. Prosecution of cult leaders led to the dispersement of the organization.

The idea of searching for "patient zero" for vaccine design has very recently become an exciting topic in vaccine design for the human immunodeficiency virus type 1 (HIV-1) which is causing the AIDS pandemic. For most outbreaks, the pathogen does not evolve very fast, so every infected individual is infected with essentially exactly the same pathogen. HIV-1 does not really evolve much faster than influenza or other RNA viruses, but it has been spreading in humans for more than 70 years, so it has developed a lot of diversity in that time. Influenza travels around the world in one year or less, and we make a new vaccine every 3 or 4 years to cover the strain that is currently circulating. HIV-1 takes dozens of years to go around the world, and we would like to make a vaccine that will cover all of the strains that are now circulating. For HIV-1, the "patient zero" is certainly not still alive, but we can reconstruct the most likely ancestral virus, by observing the currently circulating forms. This is similar to reconstructing a language such as the one spoken by ancestors of moderm Peublo Indians. For English/French/Spanish we written Latin to use. But for languages that were nevre written down, there is no such "fossil record". For current strains of influenza H1N1, which has been circling the world each year since the initial outbreak in 1918, we recently recovered an actual 1918 virus sequence from frozen samples (both victims who died of flu and were buried in permafrost, and tissue samples stored in hospital freezers sine 1918). So we can compare a theoretically reconstructed ancestor virus sequence with the actual virus sequence, and see how accurate our methods can be.

All organisms evolve with some rapidly evolving parts and some parts that do not evolve at all. For example all vertebrates have a backbone, but some have lungs and some have gills. So to build a vaccine or a drug, that is effective against a whole class of organisms, such as all HIV-1 viruses, we want to find out which parts are invariant and focus on them.

With an entirely new human outbreak, with a virus or bacteria never before seen or studied by humans, identifying "patient zero" is important for the epidemiology, but for discovering the pathogen we need to look at a group of infected people to discover what they all have in common. For the 2003 SARS Coronavirus outbreak, it took only a few months to identify the virus, and another year or two to determine that bats are the natural host (and not Civet cats as was first reported). Likewse the Sin Nombre hantavirus was discovered in just a few weeks from just a few cases.