Мне кажется, что предложенный коллегой термин "infestation level "скорее относится к зараженности клещами (или иными паразитами) животных/человека. Речь обычно идет о паразитировании -т.е. жизни за счет хозяина (вши, клещи, гельминты,пр.).

http://www.bioone.org/perlserv/?request=get-document&doi=10....
040%5B0268%3ATAIOWM%5D2.0.CO%3B2&ct=1
Ticks were collected for identification, and infestation level of animals was assessed by scoring.
Но я не знаю, можно ли сосуществование вируса и клеща можно причислить к подобным. Не думаю, что надо так глубоко "копать", поэтому предлагаю более "безопасный" термин:

http://www.nal.usda.gov/awic/pubs/westnile/1993.htm
Virus was detected by isolation, indirect fluorescent antibody, and histochemical techniques in the salivary glands, ovaries, synganglia, and coxal fluids. Infected ticks successfully transmitted virus to clean chickens on day 20 postfeeding. No evidence of transstadial transmission from nymph to adult was detected. Larvae from experimentally infected females successfully transmitted virus to clean chicks and virus was recovered from F1 larvae. Venereal transmission was not detected. Virus was present in coxal fluids secreted by infected females after infective meals. This study demonstrates West Nile virus infection in experimentally infected A. arboreus ticks and documents horizontal and vertical transmission in this species.

http://www.emedicine.com/emerg/topic584.htm
Ticks can carry and transmit a remarkable array of pathogens, such as bacteria, spirochetes, rickettsiae, protozoa, viruses, nematodes, and toxins. A single tick bite can transmit multiple pathogens, a phenomenon that has led to atypical presentations of some classic tick-borne diseases. In the United States, ticks are the most common vectors of vector-borne diseases.

http://www.ncbi.nlm.nih.gov/pubmed/13221421
Distribution of Colorado tick fever and virus-carrying ticks.
http://www.fairfaxcounty.gov/hd/westnile/wnvpamp.htm