New Report Reveals The Most Dangerous Diseases That Could Spread Like Wildfire If We Let Our Guard Down

From childhood bugs like measles and chickenpox to killer infections like Ebola, a new breakdown by a top scientist has exposed which diseases are truly the most infectious on the planet right now. It comes as the UK Health Security Agency (UKHSA) urges parents and carers not to skip vaccine appointments this summer, especially with measles cases already soaring.

Since January, there have been 674 confirmed cases of measles in England alone, and tragically, a child died from the virus at Alder Hey Children’s Hospital in Liverpool in July. The hospital has since warned that measles is rapidly spreading among young people in the region, reported the Express.

During the height of the COVID pandemic, we all became familiar with the term R0 – or R-nought – which tells us how easily a disease can spread. If the R0 is one, it means the infection level is stable. If it’s higher, it grows rapidly. For example, an R0 of two means each person infects two more – and the chain keeps growing.

Now, Dan Baumgardt, a senior lecturer in physiology, pharmacology and neuroscience at the University of Bristol, has offered a look at the most and least contagious illnesses we face today, writing in The Conversation. He explains how different diseases spread in different ways – from airborne droplets to blood, insects, or even contaminated food or water. But understanding how they’re transmitted is the first step in knowing how to protect ourselves and those around us.

Top of the list in terms of contagiousness is measles, which has made a global comeback, even in wealthier countries like the UK and US. The biggest reason? Falling vaccination rates, caused by COVID-related disruption, conflict, and growing misinformation about vaccines. Measles has an R0 between 12 and 18. That means one person could potentially infect hundreds in just two cycles of transmission. Even walking into a room two hours after an infected person has left can be enough to catch it – without ever coming into contact.

Dr Baumgardt explains that measles is so contagious that you can spread it even before you show symptoms, making isolation harder. The good news is that vaccination remains highly effective, helping to limit how many people are vulnerable in the first place.

Other fast-spreading diseases include whooping cough (with an R0 between 12 and 17), chickenpox (ten to 12), and COVID, which varies by strain but typically lands between eight and 12. Though many recover fully from these illnesses, they still carry risks of serious complications like pneumonia, meningitis, and in some cases, death.

Less contagious diseases can still be dangerous. TB, for example, spreads more slowly – with an R0 ranging from below one to four – but can be incredibly hard to treat. It needs at least six months of strong antibiotics and can affect parts of the body far beyond the lungs. Drug-resistant TB is also on the rise, making treatment even trickier.

Ebola, while not as contagious as measles, is far more deadly. Its R0 ranges from 1.5 to 2.5, and it’s spread through close contact with bodily fluids. Other diseases like MERS, bird flu, and leprosy have R0 values below one, but that doesn’t mean they’re harmless – just harder to catch.

The key message is clear: just because a disease spreads less easily doesn’t make it less dangerous. Vaccines and herd immunity still play a massive role in keeping everyone safe – especially those who can’t get vaccinated themselves.