Many of us worry about catching a bug after being around someone with a cough, finding out a friend tested positive for Covid, or hearing about our child’s classmate throwing up. With flu and vomiting bug seasons in full swing, ongoing Covid infections, and now measles outbreaks, it might feel like getting sick is inevitable. Or, when we do get sick, we try to figure out “whose fault it was,” indulging in a blame game that is common in homes and workplaces. However, not all infections spread as easily as others – it depends on various factors including the infected person’s immune system strength and the incubation period – the time it takes for symptoms to show after being infected. 
You catch cold or flu by inhaling particles of the virus, usually in a droplet of mucus from someone who is infected, via coughs or sneezes — but also from touching surfaces that have been contaminated, says Professor EcclesBut one of the most important factors is the pathogen’s reproduction rate (or R rate), a measure of how contagious it is, based on the average number of secondary infections one person causes. An R rate of 1 means that on average every person who is infected will infect one other person. Measles (caused by a virus, morbillivirus) and whooping cough (caused by a bacterium, Bordetella pertussis) are two of the most infectious diseases with the highest R rates – 15-20 and 15-17 respectively. This means one infected person can infect up to 20 others with measles, and up to 17 for whooping cough. This is why the current measles outbreaks in Birmingham and London are causing such concern, as nine in ten people who’ve been in a room with someone infected will catch it if they don’t have immunity either from past exposure or from vaccination.Tuberculosis (TB), one of the deadliest infectious diseases, has an R rate in the UK of less than 1, which means an infected person is unlikely to infect someone else – you’d need prolonged close contact to catch it, not just sitting next to someone on a bus, for instance. Bugs that spread well have certain characteristics that help them do this. For example, the bacterium that leads to whooping cough causes a cough for six to ten weeks, to maximize its spreading potential. Each bug has its own incubation period – with some infections, you will notice symptoms within a few days after exposure, but with others it can take several weeks for any symptoms to appear. For instance, measles and chickenpox (caused by the varicella-zoster virus) have an incubation period of up to three weeks. 
Most viruses infect us through the same mechanism. They enter cells, hijack the machinery to aid replication, burst out, and infect more cells, whether through nasal passages, orally or via the skin. Whether an infected person is contagious during the incubation period depends on the type of infection – with measles and chickenpox, for example, the individual may be unaware they are infected and could be spreading the viral droplets to others for weeks before their own symptoms appear. “It’s not clear why some have a longer incubation period than others, but it’s probably because they are evading the immune system,” says Ron Eccles, an emeritus professor and former director at the Common Cold Centre at Cardiff University. The amount of virus the infected person is ‘shedding’ at that moment, the environment’s temperature and humidity, as well as immunity, age, general health, and genes, all affect whether you catch an infection or not, explains Professor Eccles. Genetic factors may also play a part in whether you catch an infection and develop symptoms, adds Professor Sheena Cruickshank, an immunologist at the University of Manchester. The more immune you are to the pathogen, the less likely you are to be a good host for that virus to replicate and then spread to other people – arguing for vaccination; you’re not just protecting yourself, but others, too. We asked leading experts for the low down on the ‘catchability’ of some of the most common infections and how best to protect yourself. MEASLES WHAT IS IT: Caused by a virus, morbillivirus, this is very infectious and causes flu-like symptoms initially, and then a red, blotchy rash on the skin. SPREADABILITY: The R rate is 15 to 20. The measles virus spreads easily via droplets in coughs and sneezes. The measles virus is airborne and you can catch it even two hours after an infected person has left the room. INCUBATION PERIOD: Seven to 18 days. On average, symptoms such as a cough, fever and runny or red eyes begin after ten to 12 days — these symptoms can look very similar to a common cold. Three to five days after that, a skin rash will appear. ‘The measles rash usually starts behind the ears and spreads along the hairline and down the body,’ explain Dr Samira Neshat, a consultant critical care paediatrician at the Royal London Hospital in London and the private HCA The Portland Hospital for Women and Children. This delayed response is because it takes time for immune cells to bind to the virus and they become stuck in the skin — ‘this is when spots appear’, says Professor Eccles. HOW LONG ARE YOU INFECTIOUS? You can shed the virus and infect others from four days prior to the rash starting, to four days after the rash appears. Dr Neshat says measles can be a scary disease unless you are vaccinated, or have immunity, because 30 per cent of people can develop complications. COMMON COLD AND FLU WHAT IS IT: There are hundreds of different cold viruses, around 200, half of them are rhinoviruses (‘rhino’ means of the nose, and they are spread by respiratory droplets). There are more than 60 flu viruses – the four main strains are A, B, C and D; seasonal flu is caused by A and B strains, which mutate each year. SPREADABILITY: R rate for both is 1 to 2 but can vary. Cold and flu viruses spread easily; you’ll probably have more colds in your lifetime than any other disease. ‘This is because colds generally cause a mild illness and people go to work and school and keep spreading the viruses,’ says Professor Eccles. ‘Flu viruses are slightly more contagious as there are new ones every year, so you won’t have immunity to them and, as a result, they have more chance of successfully taking hold.’ You catch cold or flu by inhaling particles of the virus, usually in a droplet of mucus from someone who is infected, via coughs or sneezes — but also from touching surfaces that have been contaminated with these droplets (the viruses can survive for a few hours outside the body) and then touching your mouth, nose or eyes,’ adds Professor Eccles. ‘The viruses can attach to cells at the back of the nose, and that’s where you’ll often get the first signs of infection — a sore or tickly throat, for instance.’ While flu virus is slightly more infectious than a cold virus, the problem with colds is they cause a relatively mild illness and don’t provoke a strong antibody response, so you may catch the same virus again after a few weeks. INCUBATION PERIOD: 12 hours to three days of becoming infected with either type of infection. HOW LONG ARE YOU INFECTIOUS? Up to 14 days after symptoms start, or even 1-2 days before. You are most infectious in the first three days of symptoms starting, then gradually less so as your immune system response kicks in. ‘Technically you can be infected and have no symptoms and still spread it — but this is unusual,’ says Professor Eccles. NOROVIRUS WHAT IS IT: Also known as the winter vomiting bug, this is a highly contagious virus, causing vomiting and diarrhoea. It can spread rapidly through households, hospital wards, care home and schools. SPREADABILITY: R rate is 1.1 to 7.2. It only takes less than 100 particles to make you sick, and people shed billions in their stools and vomit. It is airborne, so you can become infected by breathing it in, but also from infected food and water and contaminated surfaces (it can live on hard surfaces for 12 hours; soft surfaces such as carpets for up to 12 days). INCUBATION PERIOD: 12 to 48 hours. HOW LONG ARE YOU INFECTIOUS? You are still infectious for 48 hours after your symptoms have stopped; but some studies suggest you can still spread it after several weeks. COVID-19 WHAT IS IT: An infectious disease caused by the SARS-Cov-2 virus. It can latch on to cells all around the body, causing a wide range of symptoms, from a continuous cough and shortness of breath, to gut problems such as diarrhoea and vomiting. 
Despite Covid-19 spreading easily, Professor Kucharski says: ‘There is a lot more immunity in the population now, because of vaccination and from catching the virus previously, so it will generally cause a milder illness than in 2020.’ SPREADABILITY: R rate is 1.9 to 3.9. Like many viruses, Covid spreads easily via tiny droplets in coughs and sneezes — and to a lesser extent, touching an infected surface. While it’s a respiratory infection primarily, this virus has the ability to attach to receptors throughout the body. Adam Kucharski, a professor of infectious disease epidemiology at the London School of Hygiene & Tropical Medicine, says the reason Covid spread so effectively was that people were infectious before they developed symptoms. ‘From a virus transmission point of view, that works in the virus’s favour as you have a whole bunch of people shedding a lot of the virus unaware that they are infectious.’ People might worry when they hear someone with a lingering cough in a public space, but by that stage it might be the tail end of their infection and the amount of virus coming out — if any — is lower. He adds that if one person is infected, the real-world risk of someone else in their household becoming infected is around 25 per cent. INCUBATION PERIOD: 2 to 14 days. ‘People seem to be developing symptoms much more quickly now, within two days in some cases,’ says Professor Eccles. HOW LONG ARE YOU INFECTIOUS? This varies depending on the quality and quantity of immunity you have (e.g. from vaccination or prior infection) — some people are no longer infectious after five days, but others can be infectious up to ten days, according to the NHS. Despite it spreading easily, however, Professor Kucharski says: ‘There is a lot more immunity in the population to Covid now, because of vaccination and from catching the virus previously, so it will generally cause a milder illness than in 2020.’ WHOOPING COUGH WHAT IS IT: Caused by a bacterium, Bordetella pertussis, it affects the lungs and airways, causing a cough that can last up to ten weeks. SPREADABILITY: The R rate is 15 to 17. The bacteria spread easily through the air via sneezing and coughing.INCUBATION PERIOD: 5 to 10 days. HOW LONG ARE YOU INFECTIOUS? Cold-like symptoms start after the incubation period. Two weeks later, these can worsen into coughing fits (followed by a whooping sound as you gasp for air). You are infectious from six days after becoming infected, when the flu/cold-like symptoms begin until three weeks after the coughing fits start. You can have symptoms for up to ten weeks, but not necessarily still be infectious. Treatment includes antibiotics and earlier treatment can reduce the severity of the coughing fits. Dr Neshat says: ‘It’s not always obvious if someone has whooping cough in the early stages. It can appear to be nothing more than the common cold. The main difference is that, in babies, it can cause apnoea, which are life-threatening pauses in breathing. Babies in the UK are vaccinated at eight weeks against whooping cough routinely and this is the best way to get protection.’
