In reply to Tony Jones:
I think the factors involved are the proportion of ticks carrying Lyme disease, the proportion of infected ticks that, when feeding on us, transmit the disease and the reaction of the individual to the Lyme disease agent, the number of people visiting areas with a high incidence of ticks
I think you are correct that not everyone who is infected goes on to develop the disease either, at all, or to the full extent of infection.
The abstract here https://academic.oup.com/rheumatology/article-abstract/33/2/123/1772700 is interesting.
It is from an old study and the sample of workers in Richmond and Bushey parks is small (44) but I found it surprising that only 23% reported tick bites. I would have thought it would have been much higher, but perhaps ticks were less common in 1994. Also interesting that only 3 had symptoms compatible with Lyme disease but 10 had raised levels of antibodies.
This https://www.getwestlondon.co.uk/news/local-news/deadly-blood-sucking-tick-f... is more recent but indicates the very low incidence of infected ticks in the two parks.
It would be interesting to repeat the 1994 study in Richmond and Bushey parks now.
Although Lyme disease can be very serous I think the chances of contracting it are low if you are careful, take precautions and check yourself after visiting tick areas. I have lost count of the number of ticks I have removed from me in the last 15 years or so. The only change I have seen is the ticks that hitch a ride now are tiny compared to those from years ago.
Dave